Simon Walton’s 90th-minute strike dealt what is surely the fatal blow to the Cobblers’ promotion play-off hopes as Chris Wilder’s men slumped to a 2-1 defeat at Stevenage.
Town had led 1-0 at half-time through Zander Diamond’s clinical finish, but a 65th-minute header from Chris Whelpdale and Walton’s late deflected effort ensured the visitors went home empty handed.
To pour salt into the Cobblers wounds, Ryan Cresswell was sent-off in stoppage time after picking up two bookings.
The defeat means that Town are now seven points adrift of the play-off places with just four matches to go, and althought it’s not mathematically impossible that they can still get into the top seven, they will surely be playing league two football next season.
The Cobblers made three changes to the team beaten 1-0 by Cambridge United at Sixfields on Easter Monday.
Ricky Holmes was left out to rest his troublesome knee injury, while Chris Hackett and Lawson D’Ath were dropped to the bench.
Diamond, Ivan Toney and Diego De Girolamo were the men promoted to the starting 11, with Town lining up in a 3-4-3 formation.
Cresswell, Diamond and Lee Collins made up the back three, Brendan Moloney, Jason Taylor, Joel Byrom and Evan Horwood the midfield, with the attacking three made up of De Girolamo, Toney and James Gray.
After a quiet opening, Stevenage had two great chances in the space of 60 seconds after 10 minutes.
Firstly, Bira Dembele headed wide from a deep free-kick to the far post and then, after a poor Matt Duke goal-kick had been won by the home side on halfway, Chris Beardsley broke powerfully down the right and sent in a low cross that was poked wide by Chris Whelpdale, who should have done better.
Stevenage got some joy down the sides of the Town back three early on, but they failed to make the most of their opportunities.
The Cobblers got their first meaningful shot on goal away on 18 minutes, but Toney sent a rushed snapshot well wide from 20 yards, when he had more time.
The game then degenerated into a scrappy affair, with the poor pitch, which was very patchy, bobbly and dry, not helping either side.
Stevenage were doing most of the pressing, and on 35 minutes Duke produced a great save, diving to his right to palm away Beardsley’s stinging left-foot half volley from the edge of the penalty box.
The Cobblers had barely been in the game as an attacking force, but they were ahead on 39 minutes, with Diamond the unlikely scorer.
Byrom’s left wing fee kick was scuffed somewhat, but somehow the bobbling ball evaded everybody and broke to Diamond who was lurking on the right had corner of the penalty area.
The Scotsman showed an ice-cool calm to hit a first time shot back across goalkeeper Chris Day and into the Stevenage net.
It was a quality finish from the big central defender.
That was how it stayed until the break, and manager Chris Wilder would have stressed to his side the importance of making a solid start to the second half.
That was exactly what they did, and on 59 minutes the Cobblers very nearly doubled their lead.
Receiving the ball on the left edge of the penalty area, Toney sent a shot towards goal that lacked power, but almost caught out Day who had to dive to his left to keep it out, full-back Jerome Okimo beating Cobblers substitute Lawson D’Ath to the rebound.
Stevenage equalised on 65 minutes though, and it was sloppy defending from the Cobblers.
Dean Parrett was allowed too much time to get his cross in from the left, and he picked out the unmarked Whelpdale at the far post, and the Stevenage man looped a header back across Duke and into the goal.
Going into the final 15 minutes the game began to get stretched.
Cresswell made a crucial block on a Beardsley shot to send it wide of the Cobblers goal, and on 78 minutes Town had the ball in the net, but Toney’s effort was ruled out for offside against John-Joe O’Toole who had sent in the low cross from the left. It looked a tight call.
With 10 minutes left Bruno Andrade was a whisker over the top with a 20-yard free-kick, with Duke beaten, and the same player had a great chance to put Stevenage ahead with five minutes remaining.
The home side had a three on two break, and Andrade was sent clear, but he delayed his shot, cut back on to his left foot, and hammered the ball high over the bar from the right side of the area.
With three minutes left, O’Toole had a superb chance for Town, but he hit his left foot shot straight at Day after linking well with Moloney. O’Toole should have done better.
With 90 minutes up, Stevenage scored their second, with Andrade laying the ball into the path of Walton, and his shot from the edge of the penalty area flew past Duke after taking a deflection.
To complete a miserable day for Town, Cresswell was dismissed after he deliberately handled the ball while challenging Day in the Stevenage penalty area. It was his second booking, and he was off.
Stevenage: Day, Dembele, Bond, Whelpdale, Pett, Parrett, Wells, Beardsley, Lee, Okimo, Conlon. Substitutes: Beasant, Walton, McAllister, Andrade, Yamfam, Akinyemi
Cobblers: Duke, Moloney, Cresswell, Diamond, Collins, Horwood, De Girolamo, Taylor, Byrom, Gray, Toney. Substitutes: Jalal, Carter, O’Toole, Hackett, Tozer, Perry, D’Ath
Referee: Chris Banks
Goals: 39 mins - Diamond (0-1); 65 mins: Whelpdale (1-1); 90 mins: Walton (2-1)
Cobblers fans: Approx 650