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Dartford conceded two goals in the last 11 minutes on Tuesday night as Grimsby came from behind to confirm their place in the play-offs.
After Grimsby keeper James McKeown had thwarted Dartford with some magnificent saves, Andy Cook and Aswad Thomas struck to spark celebrations among the travelling contingent.
It was a mistake which led to the opening goal on 26 minutes. Thomas, on for the injured Ian Miller, gave the ball away and Charlie Sheringham sent Nathan Collier racing into the box. Collier fluffed two chances to score but Jack Evans (pictured) made no mistake when the ball fell his way.
Darts stopper Csaba Somogyi twice denied Lenell John-Lewis after the interval, before Cook headed home a Frankie Artus cross on 79 minutes to level the scores.
McKeown saved superbly from substitute Danny Harris and Elliot Bradbrook then powered a header over when it looked easier to score.
Grimsby duly went on to snatch the winner two minutes from time. Artus again provided the assist and Thomas nodded past Somogyi.
The defeat saw Dartford drop to a place to 10th in Blue Square Bet Premier. They entertain Braintree Town on Thursday night, kick-off 7.45pm.
Reaction to Tuesday night's defeat from manager Tony Burman in this week's Dartford Messenger.