The government proposals were passed by nine votes
The DUP have voted in favour of the government's proposal to allow terror suspects to be detained for 42 days without charge.
The government won the vote in the House of Commons by 315 to 306, a majority of just nine.
Sylvia Hermon, the Ulster Unionist Party's only MP, said she also voted in favour of the legislation.
During the debate, the SDLP's Mark Durkan said the proposal was "madness". His party's three MPs opposed the move.
The DUP leadership was holding meetings with Prime Minister Gordon Brown up until 1700 BST.
It is understood the party only decided upon which way to vote after these talks concluded.
There is speculation as to what the DUP were offered in return for supporting the government.
Some have suggested the party may have been offered help with the shortfall in Northern Ireland's policing budget, while others believe a deal may have been done on the introduction of water charges.