A last-ditch try left Portsmouth beaten 27-24 at London two south west league leaders Sutton & Epsom.
The Rugby Camp outfit led 24-20 when the referee declared it was the last play.
They then managed to concede a penalty for offside and from a quick tap, Portsmouth paid a heavy price for missing a tackle.
That allowed Sutton & Epsom to cross for a dramatic winning score.
‘It was gutting,’ said Portsmouth coach Ian Chandler.
‘For 70 minutes we were the better side and it was one of our best performances of the season.’
Portsmouth were also forced to battle on without Lee Chandler in the second half.
Ian Chandler added: ‘We more than matched the top side in the league and have to take a lot of encouragement from that.
‘Unfortunately, it was missed tackles that cost us.
‘To be fair, we lost Lee Chandler to injury in the second half and he is one of our big defensive strengths.
‘We ended up with two wing-forwards playing out in the backs after having to reshuffle.
‘Afterwards, Sutton & Epsom reckoned we were the best team they have played this season but that is scant consolation.’
Portsmouth hit back from 12-0 down with a catch-and-drive line-out try from prop-forward Dan Sargeant and an Andy Barnes penalty.
Sargeant repeated his try-scoring heroics with another after the break.
Then, three further Barnes penalties saw Portsmouth stretch out into a dominant 24-12 lead.
The hosts hit back with a penalty and a try but, with the final play looming, Portsmouth still held the advantage.
Then came the last-second heartbreak to leave Portsmouth stunned at the final whistle.
Overall, despite a poor start and the late woe, Chandler was happy with his men.
He added: ‘We paid the price for not playing with the intensity required at the start of the game.
‘Then at the death we missed a vital tackle.
‘I can’t knock the performance, though.’