The DUP effectively sank the prospect of UK involvement in military action in Syria after none of its eight MPs backed intervention in Thursday night's crunch vote.
Five of the party's MPs voted against action while another three were absent from the debate where MPs rejected possible strikes against Bashar al-Assad's regime to deter the use of chemical weapons by a 13-vote margin.
David Cameron had made a last-minute phone call to DUP deputy leader Nigel Dodds ahead of the debate – but was unable to secure the party's backing.
SDLP, Alliance and independent MP Lady Hermon also voted against the Government.
It was the first time a Prime Minister had been blocked by MPs over the prospect of military action, dealing a severe blow to Mr Cameron's authority and international standing.
The Prime Minister said he would respect the will of the Commons and the Government would act accordingly.
Eight of Northern Ireland's 18 MPs were absent for Thursday night's debate: the five Sinn Fein MPs who do not take their seats and three of the DUP's representatives – David Simpson, William McCrea and Ian Paisley jnr.
The DUP's five other MPs – Mr Dodds, Gregory Campbell, Jeffrey Donaldson, Jim Shannon and Sammy Wilson – voted against military intervention.
Because of the slim margin of the Government's defeat – 13 votes – the absence of DUP support effectively scuppered Mr Cameron's hopes of securing a mandate for action. It was a reversal from 2008, when the support of the DUP's then nine MPs ensured Gordon Brown won a Commons vote on extending the maximum time police could hold terror suspects to 42 days.
Political commentator Alex Kane said the DUP could exploit a weakened Government in future debates.
"If there is a sense that the Government is seriously weakened, and if people decide to flex their muscles, then the DUP's MPs could come into play," he said.
Yesterday, Mr Dodds said the situation in Syria was "not a simple case of good versus evil".
"The Government were unable to persuade the DUP of its case on Syria," he added.
Mr Dodds said military action may only add to the problems in the region.
"The crimes of President Assad are well-known, but we also know of the atrocities carried out by the rebels, who include groups like mass murderers al-Qaida," he said.
"Any military intervention, therefore, would require taking sides in a conflict in which both sides are equally brutal, where there is a real potential of being sucked further and further into a vicious sectarian civil war, and where the outcome could be more death and destruction rather bringing peace closer.
"The DUP does not believe that it is a choice between military intervention or inaction."
The SDLP's three MPs – Alasdair McDonnell, Margaret Ritchie and Mark Durkan – also voted against military intervention by the UK.
Alliance MP Naomi Long said the Government failed to make the case for intervention, adding that the focus now must intensify on diplomatic pressure.
In June 2008, then Prime Minister Gordon Brown narrowly won a House of Commons vote on extending the maximum time police could hold terror suspects to 42 days. The proposal was passed by 315 to 306 – with votes by the nine DUP MPs proving crucial.
There was uproar in the Commons as the result was announced, with Tory and Lib Dem MPs shouting "you've been bought" at the DUP benches. They claimed the DUP was offered a string of inducements – including extra financial help for Northern Ireland – to guarantee its support.