The English champions had romped through the group stage with six wins, claiming a try-scoring bonus point in all but one of them, but they failed to turn up for the majority of a match they were expected to win at a canter.

It took a 69th minute try from Ashton to push a team parading six of England's Grand Slam winners clear, the wing switching on the afterburners and picking a clever line to cross in the corner.

Ashton's try lifted his tally since completing a 10-week ban for making contact with the eye area of Ulster's Luke Marshall to three in two games.

On the other wing, Chris Wyles plundered a late opportunist try to assure victory, while Owen Farrell finished with a 19-point haul that saw him complete every shot at goal.

The result has set up an all-Premiership semi-final against Wasps, who were last-gasp winners over Exeter earlier in the afternoon, at the Madejski Stadium on April 23.

Northampton were squeezed into submission during the final quarter, but the fact they had pushed the tournament favourites so hard was an achievement in itself given their injury crisis.

Eight front line players numbering 207 caps - among them Dylan Hartley, Tom Wood and George North - had been lost to the treatment room yet they were magnificent with backs rows Teimana Harrison and Ben Nutley outstanding.

For all their injury problems, it was Northampton who left 11 points begging during an impressive start that saw the sin-binning of Saracens openside Will Fraser for a late tackle on Ben Foden.

Stephen Myler pushed two simple penalties wide of the uprights before wing Jamie Elliott blundered by dropping the ball over the line when all he had to do was touch down. In the same move, Nutley failed to put Kahn Fotuali'i through an obvious hole.

The try eventually came in the 16th minute, however, when Harrison easily rounded opposite number Billy Vunipola, broke down the right touchline before feeding Ken Pisi for the try.

Farrell replied with a penalty but Northampton remained in control with a mazy run from Elliott launching an attack that finished when Myler steered the ball between the posts.

A forward pass from Duncan Taylor to Farrell prevented a certain Saracens try and shortly after the Scotland centre knocked on as the English champions probed the midfield.

Billy Vunipola ran over Harrison from a scrum and Petrus Du Plessis blasted a hole as the favourites made a bulldozing start to the second half, but Taylor's try was ruled out after Fraser had blocked Luther Burrell.

With a strong wind at their backs, Saracens were happy to kick down field but Northampton's mix of intelligence and determination kept them in the hunt against the odds.

Myler and Farrell swapped penalties to leave Saints 13-12 ahead, Courtney Lawes flattened England team-mate Farrell as a nerve-shredding final quarter counted down.

It was Saracens who found the extra gear when it was needed, creating the gap for Ashton to dash over before Wyles brought a sense of relief to Allianz Park. Lawes' late try ensured the scoreline was kinder to Saints.