The DUP and UUP will today support UK military intervention against so-called Islamic State in Syria.
A marathon Commons session will see MPs vote on whether to launch bombing raids on IS targets.
David Cameron has urged MPs of all parties to back his case for strikes. Although Labour leader Jeremy Corbyn has indicated he will vote against, he has allowed his MPs a free vote.
The support of 10 unionist MPs should help give Mr Cameron a clear majority, with up to 100 Labour MPs also expected to back the Prime Minister's motion.
It will allow him to press ahead with plans for a UK bombing campaign, with the first strikes expected within hours.
The SDLP plans to vote against UK intervention.
DUP MP Nigel Dodds said the Troubles had taught us that "no other approach can be brought to terrorism".
"We needed to know that the vile terrorists of ISIL/Daesh would be the target," he said.
"We had to be sure that they are a clear and present danger to the UK. We needed to be convinced that British action would make a real and practical difference. And we required a definite strategic framework being in place, including a clear exit strategy for British personnel. After repeated briefings from the National Security Council on Privy Council Terms, and much discussion with the Prime Minister and others in government, we have concluded that the time is right for us to act, and to act decisively.
"Terrorism requires an answer from all civilised countries.
"We in Northern Ireland know what it's like for terrorism to be ignored or appeased.
"The moment has come where we can no longer stand by on the other side.
"Civilians are dying, being raped or enslaved at the hands of ISIL/Daesh every day.
"These are the civilian casualties already happening and we must not, through inaction, prolong their suffering."
Ulster Unionist MPs Tom Elliott and Danny Kinahan will also support air strikes. Mr Elliott said he was satisfied military action was justified.
"While we will monitor tomorrow's debate and assess what is said by the Prime Minister and other Government representatives it is the intention that the Ulster Unionist Party Members of Parliament will be voting in favour of the motion," he said. "The party believes it meets the criteria we had outlined as necessary to support military action."
Meanwhile, North Down MP Lady Hermon said she had still to reach a decision.