Rampant West Ham booked a first League Cup semi-final for 20 years after knocking holders Manchester United out of the Carling Cup at snowy Upton Park.
Former Red Devil Jonathan Spector was in inspired form, scoring with a header and a close-range finish to notch his first goals in English football.
The disjointed visitors, previously unbeaten this season, conceded again as Carlton Cole nodded in after the break.
Cole then turned Jonny Evans to fire simply past Tomas Kuszczak.
Though Manchester United manager Sir Alex Ferguson predictably did not select his strongest team, the starting XI could still boast the likes of Ryan Giggs, Darren Fletcher and Javier Hernandez.
So it still registers as something of a shock that West Ham, bottom of the Premier League, ran out such convincing and deserved winners against the league leaders - especially in light of the Londoners' poor start to this season.
And under-pressure Hammers boss Avram Grant may now insist they are over their bad patch after a performance that was rich in promise, both individually - Nigerian striker Victor Obinna was hugely impressive as he created all four goals - and collectively.
The match started fast in freezing conditions, with both teams attacking each other as the play moved swiftly from end-to-end.
West Ham could have suffered a dispiriting early deficit but for the heroics of England squad goalkeeper Robert Green, so maligned after his disastrous World Cup campaign.
Only seven minutes had passed when Gabriel Obertan burst into space in the box to hammer a powerful low shot towards goal, but Green superbly pushed it onto the post before James Tomkins quickly hacked the ball away to safety.
The visitors briefly appeared to take control, with French under-21 winger Obertan causing Julian Faubert problems as he narrowly failed to present Giggs with a simple opportunity to score.
Though Radoslav Kovac smashed a decent chance over the bar, the Hammers appeared to be playing firmly on the back foot.
Yet Spector took charge of the match in the 15th minute as he brilliantly rampaged through the heart of the visitors' half, with Anderson's failure to challenge allowing him to set up Obinna.
The Nigerian's shot nestled in the net after an obvious deflection and Upton Park celebrated what they thought was a goal for a full minute.
But, though it looked like the ball had come off United defender Chris Smalling, the referee rightly ruled that it had skimmed off Spector - and disallowed it because he was offside.
Minutes later Spector, who made eight first-team appearances for United, gave West Ham the lead.
A clever cross from Obinna was hung up perfectly over United's back-line and the American beat Fletcher to it and looped a well-placed header past Poland's Kuszczak.
West Ham were now winning most of the individual battles and Spector added to the Londoners' advantage seven minutes before half-time.
Once again slaloming through the opposition's sleepy midfield, Spector popped a sharp pass into the feet of Obinna, who controlled it well under a heavy but futile challenge from a slipping Fabio.
Spector then sprinted to collect the loose ball before firing home emphatically from close range with both Smalling and Kuszczak unable to stop him.
The visitors tried to respond and Bebe, playing on the right, had a header plucked just from underneath the bar while Green again had to avert danger from under his own bar as Fabio's cross deflected viciously off Luis Boa Morte.
But West Ham had the best remaining chance of the first period, Obinna forcing a superb low stop from Kuszczak after Cole's knock-down.
Ferguson brought on Federico Macheda for the largely ineffective Bebe for the second half, but it made little difference as United's display remained distinctly below the standards they usually set.
Defensively they were especially lacking, and slack marking allowed Obinna to whip over a brilliant cross for Cole to nod a textbook header past Kuszczak for his 50th career goal.
Poor defending disappoints Phelan
The excellent Obinna embarrassed Rafael da Silva, on for his brother Fabio, in the 66th minute with some impressive trickery down the left wing to create West Ham's third goal.
His pass into the middle of the penalty area was controlled well by Cole, who turned Evans with remarkable ease before slotting a low shot home easily.
With the snow falling ever heavier, the white match ball was replaced with an orange one before the humbled Evans was replaced by Wes Brown in defence.
Giggs then failed to control Fletcher's clever through-ball when clear while Hernandez headed wide when well placed, summing up United's misery.
Obinna even tried an ambitious free-kick from over 30 yards before the end, displaying the confidence coursing through the veins of a west Ham side which notched their biggest win over United in 80 years.
You can watch highlights of this match on the Football League showon Wednesday at 2325 GMT online and on BBC One. The programme will then be on iPlayer and highlights will be on this website.
90:00+3:14 The game is over, as the referee blows his whistle.
90:00+2:06 Inswinging corner taken left-footed by Ryan Giggs from the right by-line to the near post, clearance by Julien Faubert.
89:16 Unfair challenge on Federico Macheda by Winston Reid results in a free kick. Ryan Giggs crosses the ball in from the free
89:16 Booking Winston Reid is shown a yellow card for unsporting behaviour.
88:34 Carlton Cole fouled by Rafael Da Silva, the ref awards a free kick. Victor Obinna has a direct shot on goal from the free
88:34 Substitution Junior Stanislas comes on in place of Carlton Cole.
86:05 Corner taken left-footed by Ryan Giggs from the left by-line, Radoslav Kovac manages to make a clearance.
85:21 Luis Boa Morte concedes a free kick for a foul on Darren Fletcher. Free kick taken by Oliveira Anderson.
83:09 Foul by Radoslav Kovac on Oliveira Anderson, free kick awarded. The ball is sent over by Ryan Giggs, Headed effort from inside
the area by Javier Hernandez misses to the left of the goal.
80:37 Ryan Giggs challenges Zavon Hines unfairly and gives away a free kick. Victor Obinna takes the direct free kick. The referee
blows for offside. Tomasz Kuszczak takes the free kick.
78:03 Unfair challenge on Luis Boa Morte by Rafael Da Silva results in a free kick. Direct strike on goal from the free kick comes
in from Victor Obinna.
76:46 Ryan Giggs fouled by Jonathan Spector, the ref awards a free kick. Ryan Giggs crosses the ball in from the free kick, Carlton
Cole makes a clearance. The official flags Federico Macheda offside. Free kick taken by Robert Green.
75:46 Substitution Zavon Hines is brought on as a substitute for Pablo Barrera.
75:29 The ball is swung over by Rafael Da Silva, clearance by Matthew Upson.
74:25 Chris Smalling gives away a free kick for an unfair challenge on Carlton Cole. Radoslav Kovac restarts play with the free
73:32 Shot from just outside the box by Javier Hernandez goes over the target.
73:32 Substitution James Tomkins goes off and Winston Reid comes on.
71:54 Substitution Wes Brown is brought on as a substitute for Jonathan Evans.
69:19 Free kick awarded for a foul by Luis Boa Morte on Javier Hernandez. Strike comes in from Ryan Giggs from the free kick, blocked
by Radoslav Kovac.
65:57 Assist on the goal came from Victor Obinna.
65:57 GOAL - Carlton Cole:West Ham 4 - 0 Man Utd A goal is scored by Carlton Cole from close range to the bottom right corner of the goal. West Ham 4-0 Man Utd.
64:07 The official flags Javier Hernandez offside. Robert Green takes the indirect free kick.
64:07 Substitution Fabio Da Silva leaves the field to be replaced by Rafael Da Silva.
60:05 John O'Shea sends in a cross, Julien Faubert gets a block in. Corner from the left by-line taken by Ryan Giggs, Radoslav Kovac
manages to make a clearance. Inswinging corner taken from the right by-line by Ryan Giggs.
59:25 Ryan Giggs concedes a free kick for a foul on Pablo Barrera. Julien Faubert delivers the ball from the free kick right-footed
from right channel, Tomasz Kuszczak makes a save.
58:57 Gabriel Obertan gives away a free kick for an unfair challenge on Luis Boa Morte. Direct free kick taken by Tal Ben-Haim.
58:38 Luis Boa Morte fouled by Fabio Da Silva, the ref awards a free kick. Tal Ben-Haim takes the free kick.
55:12 Assist by Victor Obinna.
55:12 GOAL - Carlton Cole:West Ham 3 - 0 Man Utd Carlton Cole finds the net with a headed goal from inside the six-yard box. West Ham 3-0 Man Utd.
54:52 Free kick awarded for a foul by Fabio Da Silva on Luis Boa Morte. Direct free kick taken by Tal Ben-Haim.
52:26 Chris Smalling fouled by Victor Obinna, the ref awards a free kick. Direct free kick taken by Jonathan Evans.
51:39 The assistant referee flags for offside against Carlton Cole. Jonathan Evans takes the indirect free kick.
51:05 Shot from just outside the box by Darren Fletcher goes over the target.
50:36 Inswinging corner taken left-footed by Ryan Giggs from the right by-line, Radoslav Kovac manages to make a clearance.
49:53 Fabio Da Silva delivers the ball, Pablo Barrera makes a clearance.
49:11 Fabio Da Silva crosses the ball, James Tomkins makes a clearance.
45:16 Jonathan Spector gives away a free kick for an unfair challenge on Oliveira Anderson. Oliveira Anderson takes the free kick.
45:01 The second half begins.
45:01 Substitution (Man Utd) makes a substitution, with Federico Macheda coming on for Tiago Bebe.
45:00+3:54 The match has reached half-time.
45:00+1:32 Effort from inside the area by Tiago Bebe misses to the right of the goal.
42:46 Fabio Da Silva crosses the ball, Robert Green makes a save.
41:37 Free kick awarded for a foul by Luis Boa Morte on Fabio Da Silva. Ryan Giggs crosses the ball in from the free kick, Carlton
Cole manages to make a clearance. Ryan Giggs decides to take a short corner.
41:06 Victor Obinna takes a shot. Save by Tomasz Kuszczak.
40:42 The assistant referee signals for offside against Javier Hernandez. Robert Green takes the indirect free kick.
40:22 A cross is delivered by John O'Shea, save made by Robert Green.
38:38 Carlton Cole gives away a free kick for an unfair challenge on Jonathan Evans. Chris Smalling restarts play with the free
38:11 The ball is swung over by Javier Hernandez, Tiago Bebe takes a shot. Save by Robert Green.
36:45 GOAL - Jonathan Spector:West Ham 2 - 0 Man Utd Jonathan Spector scores a goal from inside the six-yard box to the bottom left corner of the goal. West Ham 2-0 Man Utd.
34:47 Fabio Da Silva challenges Luis Boa Morte unfairly and gives away a free kick. The ball is swung over by Victor Obinna, Headed
effort from deep inside the area by James Tomkins misses to the left of the goal.
33:21 Pablo Barrera decides to take the corner short.
31:50 Ryan Giggs takes a shot. Save made by Robert Green.
30:20 Free kick awarded for a foul by Luis Boa Morte on Fabio Da Silva. John O'Shea takes the free kick.
26:50 Unfair challenge on John O'Shea by Carlton Cole results in a free kick. Darren Fletcher restarts play with the free kick.
25:32 Carlton Cole is ruled offside. Indirect free kick taken by Tomasz Kuszczak.
25:11 Victor Obinna produces a drilled left-footed shot from 25 yards. Blocked by Jonathan Evans.
24:22 Tiago Bebe takes a shot.
22:58 Ryan Giggs takes the inswinging corner, Carlton Cole makes a clearance.
21:48 Assist by Victor Obinna.
21:48 GOAL - Jonathan Spector:West Ham 1 - 0 Man Utd Jonathan Spector finds the net with a headed goal from inside the area. West Ham 1-0 Man Utd.
19:53 John O'Shea gives away a free kick for an unfair challenge on Luis Boa Morte. Matthew Upson restarts play with the free kick.
19:04 Matthew Upson concedes a free kick for a foul on Javier Hernandez. Oliveira Anderson restarts play with the free kick.
17:10 Carlton Cole is ruled offside. Free kick taken by Tomasz Kuszczak.
14:54 Javier Hernandez sends in a cross, Robert Green makes a save.
10:36 Effort on goal by Radoslav Kovac from just outside the area goes harmlessly over the target.
8:23 Tiago Bebe takes a shot. Save made by Robert Green.
Correction - 6:28 Gabriel Obertan has an effort at goal from 12 yards. Save by Robert Green.
6:28 Gabriel Obertan takes a shot. Robert Green makes a save.
3:31 Javier Hernandez gives away a free kick for an unfair challenge on Jonathan Spector. Radoslav Kovac takes the direct free
2:38 Julien Faubert is caught offside. Free kick taken by Tomasz Kuszczak.
2:00 Corner taken right-footed by Pablo Barrera to the near post, Gabriel Obertan manages to make a clearance.
1:08 Outswinging corner taken by Pablo Barrera from the right by-line played to the near post, clearance by Jonathan Evans.
0:00 The referee gets the match started.
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