Cobblers' play-off hopes suffered a potentially fatal setback when their eight-match unbeaten run came to an abrupt halt on a torrid afternoon in the Cotswolds, comprehensively beaten 3-1 by Cheltenham Town.
Without two key men in Jordan Turnbull and Aaron Pierre, Town were never at the races in Cheltenham and could have absolutely no complaints over the final scoreline, succumbing to their first defeat since losing to Colchester United way back at the start of February.
This was a return to the bad old ways as a makeshift Northampton team failed to cope with Cheltenham all afternoon at Whaddon Road, particularly the home side's front two of Tyrone Barnett and Luke Varney, who was the main beneficiary of Town's poor defending in the first-half when helping himself to two goals.
Barnett headed in a third with 17 minutes to go and that put the seal on a disappointing afternoon for the Cobblers, despite Dean Bowditch pulling a late goal back.
Northampton drop to 13th in Sky Bet League Two and nine points off Exeter City, who occupy the final play-off spot after a last-gasp win over Notts County. The Grecians also have a game in hand.
There was some surprising team news ahead of kick-off at Whaddon Road as neither Pierre (international duty), Turnbull (injured) nor Daniel Powell (dropped) made Town's starting XI.
Teenager Jay Williams returned to central midfield, Joe Powell started on the right-wing and 17-year-old Ryan Hughes made his Cobblers debut at centre-back, becoming the eighth different academy player to feature for the first-team this season.
The blow of losing both Pierre and Turnbull, two in-form players, was keenly felt during the first-half and the danger signs were evident in the 10 minutes when Cheltenham began on the front foot and twice came close to taking an early lead.
Chris Hussey blazed over less than a minute in before only razor-sharp reactions from David Cornell kept the game scoreless as he got down smartly to tip away Barnett's low shot.
Jay Williams headed straight at Scott Flinders in-between those chances, but Cheltenham were deservedly one goal to the good when Varney poked them ahead on 19 minutes.
Town should have dealt with the danger on two separate occasions in the build-up to the goal as Sam Foley gave away possession in a bad area and then Ash Taylor failed to clear Kevin Dawson's cross, giving a simple tap-in for Varney.
While the Cobblers only threatened sporadically, Cheltenham were a constant danger, especially down their right side with Ryan Broom swinging in several inviting crosses that only required a touch.
It was only a matter of time before the Robins doubled their advantage and the second goal duly arrived just shy of half-time. Again two Cobblers defenders were at fault with Charlie Goode's poor clearing header putting Hughes in trouble and he could only bundle over Barnett in the box.
Varney stepped up and made no mistake from the spot, slamming down the middle of the goal, and Cheltenham might have had a third by half-time had Barnett been able to direct Broom's latest excellent cross on target.
Keith Curle had no option but to shake things up at the break when Marvin Sordell and Bowditch were introduced for Joe Powell and Jay Williams in the hope of sparking an unlikely comeback.
Bowditch's first act was to narrowly clear the crossbar from long-range, but it continued to be an uphill battle for the Cobblers who looked every inch a patched-up team missing two key players.
The impressive Broom curled a shot just wide and Varney saw his hat-trick effort saved by Cornell, who was also called upon to thwart Dawson following Barnett's inch-perfect through pass.
The Town stopper was becoming a busy man and he also tipped over Chris Clements' free-kick, but his efforts were in vain when the home side added a third from the subsequent corner, Barnett heading in from close-range.
The final 17 minutes were mostly uneventful aside from Bowditch's well-placed free-kick, which sneaked into the corner 90 seconds from time, but that did little else than make for a more respectable full-time scoreline.
Cheltenham: Flinders, Hussey, Tozer (c), Dawson, Broom (Thomas 65), Boyle, Raglan, Tillson, Clements (Forster 89), Barnett, Varney (Bingham 86)
Subs not used: Lovett, Lloyd, Maddox, Pring
Cobblers: Cornell, Goode, Taylor, Hughes (D Powell 73), Buchanan, J Williams (Bowditch 45), Foley, O'Toole, J Powell (Sordell 45), Hoskins, A Williams
Subs not used: Coddington, Newell, Facey, Bridge
Referee: Charles Breakspear
Cobblers fans: 638