London Irish upset Aviva Premiership leaders Northampton with a 39-17 victory in their clash at the Madejski Stadium.

Jonathan Joseph inspired the biggest upset of the season so far as Irish destroyed the Saints' unbeaten record. The England centre touched down for the opening try and he played a key role in scores by David Paice and Topsy Ojo as London Irish turned the Premiership table on its head.

Jamie Gibson wrapped up the Exiles' first bonus point of the season with their fourth try to cap another outstanding individual display. By the time Ken Pisi and then Phil Dowson scored late consolation tries for Northampton, the Saints already trailed 33-3 and their goose had been well and truly cooked.

Before the match there was nothing to indicate a London Irish victory, let alone a performance like this, was on the cards from a side languishing 11th in the Premiership.he Exiles defence had conceded an average of 36 points a game up until today, with only Sale suffering a worse start to the campaign.

In complete contrast, Northampton arrived in Reading as guardians of the only unbeaten record left in the competition but they were without injured captain Dylan Hartley. Tom Wood is not back to his best and Courtney Lawes, starting at blindside flanker, was overshadowed by a dominant effort from the London Irish pack. With that platform, the Exiles back division were able to shine and no-one shone brighter than Joseph, who made his England Test debut on the summer tour of South Africa.

The England coaching staff may be back in Johannesburg this weekend to watch the Springboks play New Zealand but it will not be long until reports of Joseph's performance reach them. Both Stephen Myler and Steven Shingler had missed penalty shots at goal in a point-less first 24 minutes when Joseph created and then scored the first try. London Irish snaffled an overthrown Northampton lineout and moved the ball wide for Joseph to stab a grubber kick through for Marlande Yarde to kick on. Vasily Artemyev, Northampton's stand-in full-back, failed to cover the bouncing ball and Joseph followed up to score his second try of the season.

London Irish were unrelenting for the rest of the half. Prop Halani Aulika was stopped just before the line and number eight held up over it as Ofisa Treviranus hammered away at the line. Shingler extended the Exiles' lead with a penalty before Joseph hit the line at blistering pace inside the Northampton 22 and, two phases later, Paice twisted over the line to score.

Ian Humphreys' 40-metre drop-goal handed London Irish an 18-0 lead at half-time.

Northampton had to respond and they showed early intent after the restart with a penalty from Myler. A brilliant run from Pisi had London Irish on the back foot and Myler almost worked an opening for Noah Cato but they could not find a way through.

And London Irish then hit Northampton with a brilliant third try, finished by Ojo but created by Joseph as the Exiles attacked from clean lineout ball. Joseph beat Myler for pace and then drew two defenders before slipping a delightful pass to Ojo, who was tackled by Pisi but had the momentum to roll over the line and score.

Shingler edged the Exiles further ahead with a penalty after a powerful driving maul before captain Bryn Evans came within inches of scoring the fourth try after collecting a one-handed pass from Joseph. But the Exiles did not have to wait long. From a quick lineout, Shingler swung a long pass to Treviranus, who held off two defenders and sent Gibson racing away for the try.

Northampton, to their credit, kept plugging away with scrum-half Lee Dickson particularly lively, and they worked a try for Pisi in the right corner. Wood burrowed over for a second Northampton try but Shingler added two more late penalties as London Irish rounded off their best performance of the season with a memorable victory.

London Irish director of rugby Brian Smith was effusive in his praise of Joseph and the former England assistant coach tipped him to add to his tally of England caps in the upcoming autumn international Tests.

"I would like to see JJ get another crack," Smith said. "He is a class act and his talent is well regarded by the English coaches, and with Mike Catt (the former London Irish player and coach) now part of the England set-up, he knows what JJ can do."

The win means Irish pull further away from bottom side Sale Sharks and now enjoy an eight-point cushion as the Premiership takes a two-week break for Heineken and Amlin Challenge Cup action.

It was a big challenge for us because confidence wasn't high," Smith added. "The boys really fronted up today. They got no change out of us at scrum time and in the lineout drive and we have a lot of class in the back line."

Northampton remain top of the Premiership, two points ahead of Harlequins and Leicester. However, their director of rugby Jim Mallinder was not slow to show his displeasure at the result.

"There was no intensity there from us, our accuracy in just about everything we did that first 60 minutes was awful," he said. "Mentally, if you underestimate any team in the Premiership then you will get turned over and that is what happened. Mentally we weren't up for today`s game and that showed. We embarrassed ourselves today, we let ourselves down with a performance that was not acceptable."