Cardiff Blues (3) 11 Try: Rees Pens: Sweeney (2) London Irish (6) 12 Pens: Hewat (2), Homer, Lamb
Exiles star Steffon Armitage tackled by Gareth Thomas and Josh Navidi
London Irish condemned defending Anglo-Welsh champions Cardiff Blues to their first defeat in this competition for eight games thanks to four penalties.
England Saxons fly-half Ryan Lamb kicked the crucial points with 10 minutes remaining to seal the Exiles' first LV= Cup victory this season.
Scrum-half Richie Rees thought his try against his former club had continued their unbeaten run until Lamb's kick.
Ceri Sweeney missed a last-ditch 50-yard penalty as the Exiles hung on.
Wing Peter Hewat kicked two and Tom Homer had scored one penalty for London Irish - in replay to to former Wales fly-half Sweeney's one for the Blues - in a poor game at the Cardiff City Stadium.
But Rees' first try for the Welsh region since his move from the Exiles in 2007 was a brief highlight as the Blues capitalised on a miss-placed London Irish pass to sprint to the line from 45 yards.
The 26-year-old held off Exiles pair Alex Corbisiero and Dan Murphy to land the ball - a try that video referee Tony Rowlands endorsed - to put Dai Young's side ahead for the first time in the game.
But Lamb's 30-metre kick did not just claim the Guinness Premiership second-placed team's first win in the competition this season, but their first win over the Blues in Cardiff.
Cardiff Blues director of rugby Dai Young said: "We should have won as we had a lot of possession, territory and had enough ball. But it is a little bit of the story of our season that we have got ourselves into positions where we should be coming away with something - but we're not clinical enough.
"We are going through the phases and keeping the ball for long periods but we are not really getting behind teams and creating enough opportunities.
"Then teams are hunting down our mistakes. When we get opportunities we have to nail them.
London Irish coach Toby Booth said: "It was two sides who looked like they had never met each other before, but it depends what you want out of the game.
"I see it as an opportunity for youngsters and protecting your senior players, so from our side of things we are always going to struggle for first-phase consistency which makes attack difficult, but I was pretty pleased we were able to get over the line at the end.
"There is only one real change and that is the interpretation of the breakdown and people are terrified to take the ball into contact repeatedly, which is how you build pressure.
"The breakdown has to be a contest and a few years ago it was in the attack's favour, now it is all in the defence's favour. Hopefully it will come back to the middle.
"Until then you will see kicking and error-strewn performances. I don't want to be a part of it to be honest but those are the conditions we are under."
Cardiff Blues: Norton-Knight, Mustoe, G. Thomas, Hewitt, Czekaj, Sweeney, Rees, Filise, G. Williams, G. Powell, Jones, Morgan, White, Navidi, Rush. Replacements: T. Thomas for G. Williams (44), Hobbs for G. Powell (70), Down for Jones (59). Not Used: Blair, Evans, Allinson, Andrews, Dicomidis.
London Irish: Homer, Hewat, Joseph, Gower, Rudd, Malone, Richards, Dermody, Paice, Corbisiero, Perry, Johnson, D. Armitage, Roche, Thorpe. Replacements: Watson for Homer (40), Lamb for Joseph (56), Catt for Malone (58), Murphy for Dermody (47), Buckland for Paice (58), Garvey for Perry (74), Fisher for Johnson (34). Not Used: Lahiff.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.