Cobblers have dropped to 17th in Sky Bet League One and are now placed just a precarious four points above the drop zone after a disastrous evening which saw them lose 2-1 to Oldham Athletic at Sixfields and results elsewhere go against them.
Northampton have been teetering on the brink of a relegation battle for several weeks, but there can now be no doubt that they are firmly embroiled in a dogfight at the bottom of League One following a defeat which was both dispiriting and damaging in equal measure.
And once again it was a familiar problem that did for Northampton as Lee Erwin's stoppage-time winner snatched a deserved victory for Oldham just six minutes after John-Joe O'Toole seemed to have rescued a point.
Earlier, Tope Odebayi's opener at the midway point of the first-half, just Oldham's 10th away goal of the entire season, was the difference between the sides at half-time, something that was largely due to the home side's lack of composure in front of goal.
Latics goalkeeper Connor Ripley was to be Town's nemesis for the evening, several times denying the hosts, however he could do nothing about O'Toole's late leveller that appeared to have salvaged a precious point.
Erwin and Oldham had other ideas however, leaving the Cobblers to contemplate a hugely disappointing loss that leaves them vulnerable to several teams just below them.
With Swindon, Bury, Gillingham and of course Oldham all winning on the same evening, Northampton are now 17th, only four points above Swindon in 21st and six above 22nd-placed Port Vale, who also have three games in hand.
As the rain thundered down onto the Sixfields playing surface, Justin Edinburgh again shuffled his pack for the visit of the Latics, making two changes with Gabriel Zakuani and Hiram Boateng both recalled to the team at the expense of Zander Diamond and skipper Marc Richards.
That meant Keshi Anderson operated in a more advanced role alongside Michael Smith, and in a straightforward 4-4-2 with Matty Taylor on the left side, Northampton were quick to test visiting goalkeeper Connor Ripley who got down well to collect O'Toole's low second-minute shot.
Oldham also showed some early intent through Ryan McLaughlin's long-range effort that fizzed over Adam Smith's goal, before O'Toole had a second sight of goal not longer after, and this time he should have done better.
Anderson showed good feet on the left to drive to the byline where he picked out the Cobblers midfielder who, from 12 yards out, took a touch to make room for a shot but was unable to get enough power to beat Ripley.
Oldham were not content just to sit back and soak up pressure, though, and the visitors had the majority of possession in the opening stages, with one such spell leading to Aaron Holloway looping a header narrowly over.
Smith headed straight at Ripley from a Taylor corner at the other end but the home side were far from having it all their own way, as proved midway through the half.
After Boateng and Taylor lost possession in midfield, Oldham broke through Holloway who burst into the box and excellently picked out an unmarked Odebayi at the back post and the striker made no mistake, sweeping beyond the helpless Smith.
The Cobblers struggled to muster much of a response but it looked for all the world they would make it 1-1 on the half-hour mark when Smith met Neal Eardley's fabulous cross, only for the striker's controlled left foot volley to agonisingly sail the wrong side of the post.
In what was a ding-dong first-half, Ryan Flynn could have made Northampton pay for that miss within moments when shooting over from 12 yards before the hosts came straight back through Anderson whose shot was parried behind by Ripley.
Former Town loanee Oliver Banks then fired wide from 25 yards, followed by another presentable chance at the other end when this time Lewin Nyatanga failed to connect properly after the ball had fallen kindly to him.
O'Toole was presented with another decent opening shortly before half-time but he opted to take a touch when a shot was needed, and that brought an end to a frustrating first 45 minutes for the home side.
Ripley was back in the thick of action moments after the break, beating away a shot from Anderson for the second time, but, as in the first-half, the Latics remained a threat in attack and only a vital intervention from Buchanan prevented a certain goal.
Smith also had to beat away a shot from Flynn and Ripley did likewise from Boateng while Taylor's 20-yard free-kick clipped the top of the wall.
Sixfields grew increasingly agitated as the second-half progressed with the home side unable to build any momentum as Oldham's lead came under little serious threat.
Passes went astray and attacks quickly broke down, only adding to the crowd's frustrating at the clock ticked towards the 80-minute mark with Town having completely lost their way.
But they did carve out two good chances in quick succession with Taylor denied by a last-ditch block and then O'Toole diverting Marc Richards' cross off target.
Smith had to pull off a terrific diving save to deny Bury's Chris Taylor at the other end, and that was a moment which would proved absolutely crucial within seconds.
It seemed Oldham were going to hold on but, with six minutes to go, the man who has saved Northampton so many times over the past 18 months came to the rescue again.
The visitors failed to deal with Eardley's looping cross and after Ripley saved Richards' venomous volley, the ball rolled across the line and O'Toole won the race to force into the net to seemingly rescue a much-needed point.
But if Northampton thought that would be enough, they had another thing coming.
From the moment of O'Toole's goal, Oldham went back on the attack and looked far the likelier to take the spoils, something which they did in the first minute of stoppage time.
Substitute Ewrin was the scorer, cutting inside and rifling an excellent shot into the bottom corner of Smith's goal from 20 yards to earn a vital win and condemn Northampton to a damaging defeat.
Cobblers: Smith, Eardley, Zakuani, Nyatanga, Buchanan (c), Taylor, Wylde (McCourt 74), Boateng (Beautyman 84), O'Toole, Smith (Richards 63), K Anderson
Subs not used: Cornell, Phillips, Diamond, P Anderson
Oldham: Ripley, Gerrard, O'Neill (Fane 78), Flynn (Taylor 68), Banks, Holloway, Hunt, Clarke (c), McLaughlin, Green, Obedayi (Ewrin 68)
Subs not used: Law, Kettings, Ngoo, Edmundson
Referee: Brett Huxtable
Oldham fans: 197