Brentford boss Andy Scott was bitterly disappointed as his side were consigned to a third successive defeat on the road by Peterborough.
Bees midfielder Marc Laird looked to have earned his side a valuable point when his 74th-minute strike cancelled out Grant McCann's early penalty, but Jonathan Obika handed Posh all three points with an injury-time strike to make it 2-1.
"After starting poorly and conceding an early penalty I was pleased with the way we came back into the picture, Scott said. "We should have got something from the game but can't afford to give the ball away in crucial areas. We did that in the build-up to their late winner and paid a heavy price for it."
He added: "We played well as a team only to let ourselves down after some long spells when we were good on the ball and put Peterborough, one of the better sides in the division, under a lot of pressure.
"But we have to keep replicating the good parts of our performances and put a stop to the habit of conceding silly goals which has cost us again in this game."
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