Cardiff's bid for automatic promotion was seriously dented by struggling Peterborough who wiped away their relegation fears with a famous win.
Goals in either half from Matthew Gill - his first of the season - and Leon McKenzie left Bluebirds boss Lennie Lawrence admitting their fate will be decided by a last-day showdown at promotion rivals Crewe.
Cardiff would have jumped above Dario Gradi's side into second place if they had managed to pocket a third away win in just a week, but their failure to triumph at London Road leaves them facing a tough run-in.
Lawrence said: "I'm resigned to the fact we are going to have to go to Crewe to win to have any chance of going up automatically - you don't have to be Einstein to work that out.
"But we have three tough games against Colchester, Bristol City and Wigan before then and the lads need to raise themselves to make sure we are no more than three points behind Crewe by the time we go there.
"Tonight we started slowly and paid the price. We allowed Peterborough to get into the game and then they hurt us." Cardiff had dominated the opening half until McKenzie left two defenders trailing on a surging left-wing run before delivering a cross that was finished immaculately by Gill from the edge of the box.
And Cardiff were again controlling the second half without making a breakthrough until McKenzie pounced on some dithering by Chris Barker, burst into the box and fired an unstoppable shot into the roof of the net.
Bluebirds substitute Gavin Gordon was lucky to avoid a red card after ending Ian Hendon's match with an awful late tackle, but he and prolific strike partners Robert Earnshaw and Peter Thorne could make no impact in front of goal.
And Cardiff's night was summed up when skipper Graham Kavanagh steered a free header wide in the dying moments.
Posh chief Barry Fry said: "My players were truly magnificent and they deserve all the praise they will get. Everyone ran their hearts out and their socks off and all the fans were brilliant."