Last minute winner sees the team through.
And yet, the U's kept their heads and in a lively conclusion to the game, grabbed the winner as George Moncur slid home a delightful finish to send the Weston Homes Community Stadium into bedlam.
The drama wasn't over there, though, as the visitors hit the crossbar from the resulting corner with a thumping shot.
However, Sam Walker used every bit of his height to turn the ball away with a sensational stop to his right.
After an evenly matched 85 minutes, it looked like the game might slip away from the U's when they conceded a penalty at the death.
From despair to delight in the space of a few minutes - the FA Cup was up to its usual tricks on Sunday afternoon as Colchester United made it through to the Third Round of the historic competition. After an evenly contested battle for possession in the opening quarter to the game, it was Sam Walker first called into action as Aaron McLean latched onto a corner set piece but fired the ball straight at the keeper. The two players were to come face to face as part of that dramatic end, later in the game.
The U’s were next to create as David Fox controlled the ball on the edge of box and drew his man close, before releasing the ball to George Moncur who curled a shot that sent Ben Alnwick diving down to his right.
Behind Watt on the left hand side, Cole Kpekawa was having equal success beating his man, with the one of a few good crosses finding Tom Eastman inside the box, his volley soared well over the bar though.
The opening half draw to a close goalless, with both teams having risked little in attack perhaps a result of the added cup pressure, but with U’s playing well despite missing several players.
However the patient play of the first half was nearly undone instantly, as Eastman gifted McLean a chance on goal but then got back in time so slide in front of Walker and block the shot.
Peterborough who like U’s have no win in their last five league games, took a hold on the game following this chance, with the ball seemingly stuck in Colchester’s own half.
But as Boro's own clever set piece created a chance, Colchester blocked and countered with great play between Gavin Massey and Moncur, bringing the home side back into the game.
The U’s began to string together some possession and through the same pairing of Massey and Moncur they created a chance that saw Fox power a shot at goal, only for a deflection to send the ball wide.
Ipswich loanee Elliott Hewitt was next up for U’s, as he drove inside from right enough spaced opened up for him to hit a left footed effort at goal, Alnwick was equal to it.
As the Colchester pressure, built Tony Humes made his first change which saw Massey replaced by Healey, injecting some much needed pace into the attack and leading to an array of chances.
Two of which saw the South Stand fans screaming for handball as Borough players put their bodies in the way to block with Colchester peppering the goal.
But as the game meandered towards its end a sudden turn of events made for more action in the final minutes than anyone could have anticipated.
Watt, tracking back, fouled his man in the box, resulting in an a penalty for Posh with just two minutes to go, mirroring U’s FA Cup exit last season, but this team Walker dived right to deny McLean.
Then in typical FA Cup fashion, Colchester brushed themselves off and attacked, and when the ball fell to the hardest working player on the pitch, Moncur made no mistake in scoring a late, late winner.