Northampton threw the formbook out of the window as they produced one of the biggest shocks in Heineken Cup history at Estadio Anoeta on Sunday.

Bookmakers gave struggling Northampton a 20-point start in the quarter-final tie against last season's runners-up in San Sebastien, but they got it all wrong in a scrappy game that had nothing for the purists, but plenty for Saints fans.

There was no score after 39 minutes of poor rugby and it was left to the boot of Dimitri Yachvili to edge his side in front, only for Robbie Kydd to stun home fans with a match-winning 60-metre intercept try.

With winger Phillipe Bidabe and scrum-half Yachvili looking dangerous, Saints spent much of the opening half hour on the back foot and their cause was not helped when inspirational winger Sean Lamont was helped from the field with a shoulder injury.

Tempers boiled over as the match approached half time, with scrum-halves Yachvili and Mark Robinson going toe to toe after yet another promising Biarritz backline move ended with a poor pass and a knock-on.

The French club eventually gave up on the idea of scoring five points, opting for goal instead, but even that did not come off as Yachvili's attempt from in front drifted off target.

The France international was given another chance on the stroke of half time and made no mistake to give the home side a 3-0 lead.

Saints' fly-half Carlos Spencer should have levelled the scores early in the second half, but his infamously wayward goal-kicking came back to haunt the visitors.

A break from Mark Robinson should have resulted in a Saints lead, but the move broke down in midfield and the game limped into the final quarter.

Northampton began leaking penalties and Yachvili finally converted one to double his side's lead.

But just as the French club pushed for a third score, Saints centre Kydd intercepted a Yachvili pass and ran 60 metres to turn the match on its head.

Spencer's conversion gave Saints a shock lead and Biarritz fans were stunned into silence.

Biarritz had the chance to steal it in the dying minutes, but backrow Imanol Harinordoquy spilled a Yachvili cross-kick with plenty of support on hand and the try-line begging to be crossed.

A desperate drop-goal attempt was charged down by a wall of Saints defenders and Biarritz's final two attacks both saw the ball knocked on.