Championship: Jersey Reds 15-11 London Irish
London Irish suffered their first league loss of the season, as Jersey celebrated one of the club's greatest-ever results.
Scott Steele's try put the Exiles ahead but Jersey winger Nick Scott touched down as the sides were level at 8-8 by half-time.
Irish had won all 17 of their Championship games and led narrowly going into the closing stages.
But Gary Graham's late try from close range was enough for the Jersey win.
The victory was their ninth in a row and, though they remain fifth, it boosts their chances of sneaking into the play-offs.
Scrum-half Steele had opened the scoring just before the 15-minute mark after a series of pick-and-goes near the line, but Jersey were level when winger Scott was found by Joel Dudley in space out wide on the left.
Former Saracens full-back Ben Ransom calmly slotted over a drop-goal to edge London Irish back ahead but Callum Sheedy's penalty made it all square at the break.
Jersey knew they would be playing against a strong wind in the second period against a side used to winning and backed by about 1,000 travelling supporters in the biggest-ever crowd for a Championship fixture at St Peter of 3,317.
London Irish centre Theo Brophy Clews knocked over a penalty to re-establish a narrow advantage before the hosts found a new gear, pushing for a winning try which came when flanker Graham powered over with 77 minutes showing on the clock in the ground.
Sheedy's conversion put the Reds more than a penalty ahead, before late Irish pressure which Jersey just about withstood to claim a memorable victory.
London Irish are already guaranteed to finish top of the Championship table, while the islanders remain in fifth spot with just two games against play-off rivals Doncaster and bottom side Richmond to come.
The teams meet again in eight days' time at the Madejski Stadium in the semi-finals of the British and Irish Cup.
Reds captain James Freeman told BBC Radio Jersey: "To beat a team like that, for them to come here unbeaten and we're on a good bit of form, it's just a massive step in the right direction for us in our play-off campaign.
"There were some crucial points in the game where we were grinding it out and showing what we can do so it was huge.
"The atmosphere here today was absolutely fantastic and if we can replicate that in the coming home games it would be fantastic."
Jersey: Cuthbert; Adair, Davies, Ma'afu, Scott; Sheedy, Dudley; Woolmore, Buckle, Kerrod, Phillips, McKern, Freeman (capt), Graham, Haining.
Replacements: Selway, McCarthy, Armstrong, Kolo'ofa'i, Argyle, Hardy, Eastwell.
London Irish: Ransom; Lewington, Hearn, Clews, Ojo; Marshall, Steele; Court, Paice (capt), Hoskins, Robson, De Chaves, Narraway, Cowan, Treviranus.
Replacements: Porecki, Hobbs-Awoyemi, Chawatama, Sexton, Nayalo, Allinson, Tikoirotuma.
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