Ashton's injury is a setback for Saints

Northampton will be hoping that three is their lucky number. They ensured yesterday a third successive appearance in the Premiership semi-finals as a burst of second-half energy carried them clear of Worcester's ability to live off scraps and anything Exeter might have achieved at Saracens. However, a similar display will not suffice when they meet Harlequins on Saturday at the Stoop.

Sufficient unto the day. Northampton have enough to worry about, given that Chris Ashton must recover from the back spasm that forced him off midway through the first half. Were the England wing to miss the semi-final, he would join such unavailable luminaries as Tom Wood, Courtney Lawes, Dylan Hartley and Calum Clark and though Jim Mallinder, Northampton's director of rugby, is optimistic, Ashton's injury is an ongoing problem.

Mallinder rejected suggestions that Ashton had hurt himself with an over-exuberant dive to score the first of his side's five tries. The "Ash-splash" has not been seen much this season but Ashton has now scored in Northampton's last three games, an essential part of the momentum which means they will travel in hope to London next weekend.

"It [Ashton's celebration of his try] doesn't help but maybe he wanted to show it to Franklin's Gardens for the last time," Mallinder said. Ashton leaves for Saracens after this season but having scored 93 tries in five years in the Midlands, he would love to depart on a high. He received generous applause as he hobbled off, though not so raucous as that showered on two departing Irishmen, Roger Wilson and James Downey, during their last home game.

They know as well as anyone that Northampton cannot afford so uneven a display against Quins as this: Worcester were in the game until Shaun Perry, their captain, was sent to the sin-bin just before half-time. They had endured sustained pressure midway through the half before conceding 13 points in Perry's absence.

They pulled back again to 23-14 but ignored a straightforward penalty opportunity to close the gap still further. Northampton flicked a switch and discovered the fluency which had escaped them for the previous hour; in the final 22 minutes they scored three tries and destroyed Worcester's set-piece game, notably the line-out where James Craig, a young Leeds product, added to the laurels he has acquired this season.

Perhaps the carelessness was part of the end-of-term spirit or the wintry conditions but after Ashton finished off the first try with only five minutes played, Northampton found the tryline an almost impossible hurdle. They allowed Chris Fortey, the Worcester hooker playing his final game, to make the hard yards before Tom Arscott sent Josh Drauniniu over.

The one player to sustain the highest of standards for Northampton was Ben Foden, whose kick and chase established the position from which Perry received his yellow card. The full-back then erupted between Danny Gray and Alex Grove to score Northampton's second try and with Stephen Myler adding penalties either side of half-time, an eighth capacity crowd of the season seemed to be planning a semi-final journey.

But Worcester won a scrum against the head near their own line, then Jonny Arr stole off with another Northampton ball that squirted sideways from a scrum 15 metres out. Though the scrum-half could not score, Kai Horstmann drove over.

"They were definitely rattled," said Richard Hill, Worcester's director of rugby. But that was all they were.

With Ryan Lamb pulling some expert strings, Northampton put the match to bed: Paul Diggin skipped over, the bonus-point try fell to Teimana Harrison, after Worcester lost the ball behind a lineout, and Alex Waller was mauled over. Foden had time to make a try-saving tackle on Miles Benjamin, but he may have to work harder on Saturday.

Northampton Saints B Foden; C Ashton (T May, 19), G Pisi, J Downey, P Diggin; S Myler (R Lamb, 50), L Dickson (capt; M Roberts, 59); S Tonga'uiha (A Waller, 62), A Long (R McMillan, 59), B Mujati (P Doran-Jones, 54), M Sorenson (T Harrison, 57), C Day, J Craig, R Wilson (B Nutley, 67), P Dowson.

Worcester Warriors T Arscott; M Benjamin, A Grove (C Hayter, 68), R Fatiaki, J Drauniniu; D Gray (J Carlisle, 50), S Perry (capt; J Arr, 46); G Porter (C Jones, 15), C Fortey (E Shervington, 54), B Douglas (R Furniss, 68), J Percival, C Gillies (B Gulliver, 59), N Best (S Betty, 59), K Horstmann, M Kvesic (Fortey, 79).

Referee G Garner (Warwickshire).

Northampton Saints

Tries: Ashton, Foden, Diggin, Harrison, Waller

Cons: Myler 2, Lamb 2

Pens: Myler 2, Lamb

Worcester Warriors

Tries: Drauniniu, Horstmann

Cons: Gray 2