The Reds' survival hopes were left hanging by a thread following an Easter Monday loss against relegation rivals Peterborough United.
A Posh goal in either half condemned Barnsley to a first home defeat in seven and leaves the Reds hoping for a miracle in terms of retaining their second tier status.
The afternoon had begun quite promisingly, when inside the opening 60 seconds, Domingos Quina fired against the crossbar, before Romal Palmer ghosted into the box and forced Posh 'keeper David Cornell into a smart save, low down to his right.
Callum Styles was next to work room just outside the box and test Cornell, but his shot was straight at the visiting goalkeeper. But Cornell was worked a lot more soon after as he acrobatically tipped Carlton Morris' header onto the bar and out of play.
It seemed a matter of time before Barnsley forged ahead, but the away side grew into the contest and and Joe Ward was slipped in expertly down the right by Jack Marriott and Reds' stopper Jack Walton did well to deny the wide man with his legs.
And on 25 minutes, Peterborough snatched the lead when the Reds failed to maintain possession in midfield, the ball was worked into the box to the left where Marriott pounced with a clinical finish to Walton's right.
HALF-TIME: Barnsley 0-1 Peterborough
The interval saw Cauley Woodrow introduced for Rémy Vita and a change in system for Poya Asbaghi's side, as they looked to get back into the game.
Barnsley went close inside the opening stages of the second period when Woodrow played Styles in, one-on-one with Cornell but a great defensive challenge scuppered that opportunity, and then Morris was thwarted by Cornell on the follow-up.
Woodrow shot straight at the goalkeeper from just inside the box, whilst more desperate Posh defending saw all of Quina, Morris and Amine Bassi denied by last-ditch sliding challenges inside the penalty area.
On 75 minutes, Posh doubled their lead. A corner kick that was well won by substitute Ricky-Jade Jones was floated into the six yard box, and Frankie Kent rose unopposed to nod into the bottom corner and kill any lingering hopes of a Reds comeback.
There was another nearly moment for Morris who looked almost certain to pull a goal back but was somehow denied once more by the impressive Cornell in the United goal.
Barnsley are now 11 points adrift of safety with 12 left to play for.
FULL-TIME: Barnsley 0-2 Peterborough
Walton, Brittain, Andersen, Kitching, Vita (Woodrow), Palmer (Wolfe), Gomes, Styles (Adeboyejo), Bassi, Quina, Morris.
Unused: Jinadu, Moon, Hondermarck, Leya Iseka.
