London Irish (10) 23
Tries: Hala'ufia, Seveali'i Con: Malone 2 Pens: Malone 3
Saracens (16) 19
Tries: Melck Con: Jackson Pens: Jackson 3 Drop-goal: Goode
Samoan centre Elvis Seveali'i celebrates his match-winning try
London Irish came from behind to inflict a first Premiership defeat of the season on leaders Saracens in an absorbing game at the Madejski Stadium.
Elvis Seveali'i grabbed the decisive try on 69 minutes to seal the win and put the Exiles second in the table.
Justin Melck for Saracens and the hosts' Chris Hala'ufia traded first-half tries, before three Glen Jackson penalties took Saracens in ahead.
But Chris Malone kicked two second-half penalties to spark the Irish comeback.
Saracens, who went into the match with nine league wins and one draw, needed to adopt a high intensity to cope with the Exiles' ferocious tackling at the ruck.
But Sarries battled gamely and reaped immediate rewards as fly-half Alex Goode sent a 40m drop-goal sailing between the posts inside the first five minutes.
The visitors extended their lead when Hugh Vyvyan charged down Alfredo Lalanne's attempted clearance and Melck was quick to capitalise and go over for the first try.
Jackson added the extras to hand Sarries a 10-0 lead with just 10 minutes gone.
Irish responded immediately, putting Sarries under pressure straight from the restart, and they scored a neat try as Hala'ufia benefitted from good work by Steffon Armitage before the Tongan went over under the posts.
Malone added the extras to reduce the deficit to a single score.
Jackson added two more penalties for the visitors in the first half, while Malone converted one penalty and missed another for Irish, who finished the half strongly but went into the break trailing by six points.
Malone and Jackson again traded penalties in the second period as Saracens held onto their lead until 10 minutes from the end.
The visitors wasted several opportunities to extend their advantage by attempting failed drop-goals.
They were then punished by Seveali'i who broke through three tackles to score the decisive try that put Irish ahead for the first time.
Malone added the extras to ensure the win and move Irish to within three points of their opponents at the top of the Premiership table.
London Irish rugby director Toby Booth:
"That was a good advert, a tough contest with plenty of ambition, and was also quite entertaining. We were behind for long periods, and we had to work hard for it.
"What makes me proud is that we came back from 10 points down and never let them get away from us.
"Saracens are a pragmatic team, hard to beat, because they offer you very little, and they know how to take scores and squeeze you.
"But we won the kicking contest out there, and I was delighted with the whole of our back three."
Saracens rugby director Brendan Venter:
"The boys are very disappointed. They played well, but we will learn from this experience and become a better team because in the end we beat ourselves.
"The first try we conceded was soft, and the second was stupid, because we were not in any trouble, but simply made mistakes.
"What disappoints me is that we were a little bit asleep at times, which we have not seen before this season.
"But we were not dominated in any area of the game, and while the lads are hurting now, on Tuesday this game is history."
London Irish: D. Armitage, Homer, Seveali'i, Mapusua, Hewat, Malone, Lalanne, Dermody, Paice, Ion, Kennedy, Casey, Thorpe, S. Armitage, Hala'ufia.
Replacements: Catt for Mapusua (74), Murphy for Dermody (70), Coetzee for Paice (74), Rautenbach for Ion (62), Stowers for Thorpe (58), Roche for Hala'ufia (58). Not Used: Rudd, Lamb.
Saracens: Goode, Penney, Ratuvou, Barritt, Wyles, Jackson, Marshall, Aguero, Ongaro, Skuse, Borthwick, Vyvyan, Van Heerden, Melck, Joubert.
Replacements: Haughton for Penney (70), Gill for Aguero (40), Brits for Ongaro (44), Nieto for Skuse (28). Not Used: Botha, Saull, Rauluni, Hougaard.
Ref: Dean Richards (RFU).