Posh were beaten 3-1 in the opening group game of the Vertu Trophy as Leyton Orient secured an away win.
Peterborough United's defence of the Vertu Trophy began with a disappointing 3-1 defeat to fellow Sky Bet League One side Leyton Orient at the Weston Homes Stadium with the O's recovering from going down to 10-men in the first period to secure a notable away success.
Posh created plenty of opportunities, but were undone in transition by the O's after the interval and the tough start to the 2025/26 campaign continued for Darren Ferguson's men.
Dom Ballard fired wide twice inside the opening quarter of an hour, while Blackmore kept out Sonny Perkins with his legs.
Orient were rightly reduced to 10-men midway through the first half as Abdulai was shown a straight red card for an ugly looking high challenge on skipper Carl Johnston.
Donay O'Brien-Brady should have opened the scoring after forcing his way into the box, but he fired into the side-netting and moments later, the visitors were ahead as WELLENS produced a bit of magic from a dead ball situation.
Harley Mills and Johnston were both denied by Cahill before Gustav Lindgren squandered a chance to score after rounding the goalkeeper, but he fired over.
MILLS did level the scores in stoppage time, his left-foot shot forced over the line by a retreating defender.
Substitute Jimmy-Jay Morgan sent an acrobatic overhead kick over the bar and headed wide in the early part of the second period before MOORHOUSE made it 2-1 to the visitors with a composed finish from the edge of the box.
Khela and Mills both sent shots wide before Orient found a third goal as CRAIG waltzed through to find the back of the net as Posh were again done on the counter-attack.
Morgan fired wide of the target late on, but Posh failed to capitalise on their extra man and suffered defeat on Matchday one of the trophy.
Posh: Blackmore, Johnston (sub Hayes 46min), Lolos (sub Lisbie 58min), Khela (sub Collins 75min), Lees (sub Mendonca 46min), Nevett, Lindgren, O'Brien-Brady, Mills, Leonard (sub Morgan 46min), Dornelly. Unused subs: Smith, Collins, Andrews.
Leyton Orient: Cahill, Simpson, El Mizouni, Mitchell (sub Bakinson 75min), Craig, Edmunds-Green, Perkins (sub Koroma 65min), Moorhouse (sub O’Neill 65min), Abdulai, Wellens (sub James 66min), Ballard (sub Clare 46min). Unused subs: Phillips, Adaramola.
Half-time: Peterborough United 1-1 Leyton Orient
Full-time: Peterborough United 1-3 Leyton Orient
Attendance: 2,458 (165 Leyton Orient fans)