All three of the spinners in contention for a Test place - Monty Panesar, Graeme Swann and Adil Rashid - have been given a chance to prove themselves.
National selector Geoff Miller said: "Michael Vaughan will, of course, be disappointed not to have been included. But I met with Michael recently and he understands that he needs to make runs consistently in County cricket if he is to force his way back into the squad.
"The door has not been closed on any player at this stage."
Australia captain Ricky Ponting said the pair's exclusion did not come as a shock to him.
"I'm not surprised those two guys aren't around with the way their current group have been performing of late," he said.
"I think it pretty much picked itself to tell the truth."
Vaughan has scored only 159 first-class runs at an average of 19.88 in six matches this summer, with a top score of 43 against Worcestershire last week.
And he has also struggled with injury after suffering a recurrence of the problems to his right knee which sidelined him for over a year and forced him to miss the recent championship match with Somerset.
Harmison may consider himself unlucky to have not been included after the paceman responded to being dropped ahead of the one-sided Test series win over the West Indies by claiming 30 championship wickets in six matches, including 16 in the last two games, against Lancashire and Warwickshire.
However, the Durham bowler has been included in the England Lions line-up to face Australia in a four-day match starting on 1 July, the same day as the Warwickshire fixture.
Miller added: "We are keen to make the match at Worcester a tough one for Australia and plan to give all the players in the Lions team the maximum opportunity to press their case for selection.
"As a consequence, we will now be announcing our squad for the first Ashes Test on Sunday, 5 July - 24 hours later than originally planned."
THE ASHES SERIES
8-12 Jul: 1st Test, Cardiff
16-20 Jul: 2nd Test, Lord's
30 Jul- 3 Aug: 3rd Test, Edgbaston
7-11 Aug: 4th Test, Headingley
20-24 Aug: 5th Test, Oval
One player looking to force his way back into the reckoning is Bell, who lost his place after the debacle in Jamaica this winter but has scored consistently in first class cricket, averaging 64.5 after seven matches.
He will captain the England Lions side, which will feature Bresnan and Onions, who impressed during the second Test win at Chester-le-Street over West Indies, and have been given another chance to show what they can do.
Yorkshire's Rashid will also be looking to give a strong performance for the Lions as he seeks to continue to apply pressure on Panesar for a Test place.
Panesar has been in poor form recently for Northamptonshire, recording a worrying return at Cardiff last week of 2-149 from 44 overs.
He has claimed only six wickets in six County Championship matches this summer at an average of 86.67.
And Rashid, who impressed in the World Twenty20, has emerged as a shock contender to make the final 13-man Ashes squad.
"Rashid's inclusion reflects our view that he is maturing all the time as a cricketer and has the potential to develop still further," said Miller.
England Test squad for the pre-Ashes camp:
Andrew Strauss (captain) (Middlesex), James Anderson (Lancashire), Ian Bell (Warwickshire), Ravinder Bopara (Essex), Tim Bresnan (Yorkshire), Stuart Broad (Nottinghamshire), Paul Collingwood (Durham), Alastair Cook (Essex), Andrew Flintoff (Lancashire), Graham Onions (Durham), Monty Panesar (Northamptonshire), Kevin Pietersen (Hampshire), Matt Prior (wk) (Sussex), Adil Rashid (Yorkshire), Ryan Sidebottom (Nottinghamshire), Graeme Swann (Nottinghamshire).
England team v Warwickshire at Edgbaston (1-3 July):
Andrew Strauss (captain), James Anderson, Stuart Broad, Ravi Bopara, Paul Collingwood, Alastair Cook, Andrew Flintoff, Monty Panesar, Kevin Pietersen, Matt Prior (wk), Graeme Swann.
England Lions v Australia at Worcester (1-4 July):
Ian Bell (captain) (Warwickshire), Tim Bresnan (Yorkshire), Joe Denly (Kent), Steven Davies (wk) (Worcestershire), Stephen Harmison (Durham), Sajid Mahmood (Lancashire), Stephen Moore (Worcestershire), Eoin Morgan (Middlesex), Graham Onions (Durham), Adil Rashid (Yorkshire), Vikram Solanki (Worcestershire).
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