Peterborough United lead Town 2-0 at half-time, Ivan Toney having netted a penalty before Sammie Szmodics profited from a Will Norris error.
Cole Skuse returned to the midfield for Emyr Huws in an otherwise unchanged Town side, the Welshman having suffered an ankle injury at Rotherham in the week.
Norris continued in goal behind a back three of skipper Luke Chambers, James Wilson and Luke Woolfenden. Luke Garbutt and Gwion Edwards were the wing-backs.
In midfield, Skuse joined Flynn Downes with Alan Judge ahead of them behind front pair Will Keane and Kayden Jackson.
Teddy Bishop was missing from the bench having suffered a knee problem in training, while Peterborough were unchanged for the fourth successive match.
The visitors struck the game’s first shot in the fourth minute but Posh skipper Mark Beevers’s well-struck volley from a half-cleared corner from the left was, rather fortunately for the Blues, hit straight at Norris in the Town goal.
But Town soon began to get on top. However, their first effort at goal didn’t come until the 13th minute when ex-Posh wideman Edwards cut in and hit a left-foot strike over.
The Blues were forced into a change in the 16th minute when Garbutt, who had undergone treatment a few moments earlier, was forced off and was replaced by Myles Kenlock.
Peterborough started to look the greater threat, winning a number of corners, and on 19 Siriki Dembele hit a shot from the edge of the box which was easily snaffled by Norris.
In the 22nd minute Peterborough took the lead from the penalty spot. Dembele was played in on goal on the right of the area by Toney and Wilson slid in, catching the striker. Dembele managed an effort at goal as he fell which was stopped by Norris and then cleared by Chambers.
The Peterborough players protested but referee Andy Woolmer wasn’t interested until he spoke to his assistant and pointed to the spot. Under the old rules Wilson would have been red-carded for a professional foul but given that he made a genuine attempt to play the ball the defender rightly stayed on.
League One top scorer Toney took the kick and rolled it to Norris’s left and into the corner of the net.
On 32 Edwards crossed from the right and Kenlock headed well wide as Town looked for an equaliser but two minutes later they conceded a disastrous second.
Skuse played the ball back to Norris under no pressure and the keeper tried to take it past Szmodics just to the right of his goal but was dispossessed and the former Colchester man was able to tap the easiest goal he will ever score into an empty net.
Town looked to hit back in the 37th minute when Edwards made a mazy run down the right. The Welshman’s cross fell to Judge on the edge of the six-yard box but the Irishman was only able to stab into Posh keeper Christy Pym’s arms.
Norris had another shaky moment on 39, failing to hold on to a cross when under pressure on the edge of the box but fortunately the ball fell safely. At the other end, Jackson was hauled to the ground by his neck by Beevers but with referee Woolmer not spotting a foul.
Neither side were able to seriously threaten in four minutes of additional time and the half-time whistle was greeted by muted boos from the frustrated Town support.
Having conceded the opener via the penalty - which looked a correct decision - the Blues had started to show signs of pushing for an equaliser when Norris was undone by overconfidence inside his own six-yard box, not the on-loan Wolves keeper’s first error in recent games.
As half-time approached Town had begun to control the game against a Peterborough side content to sit in their half but without creating a significant opportunity.
The Blues will need to pull a goal back soon after the restart if they’ve any chance of taking anything from the game.
Town: Norris, Chambers (c), Woolfenden, Wilson, Edwards, Downes, Skuse, Judge, Garbutt (Kenlock 16), Keane, Jackson. Subs: Holy, Donacien, Nolan, Dozzell, Norwood, Sears.
Peterborough: Pym, Thompson, Beevers (c), Kent, Ward, Taylor, Brown, Butler, Szmodics, Toney, Siriki, Dembele. Subs: Chapman, Eisa, Knight, Bennett, Boyd, Mason, Jade-Jones. Referee: Andy Woolmer (Northamptonshire).