Lawmakers in Britain's House of Commons voted overwhelmingly on Friday to join US-led air strikes on Islamic State (IS) targets in Iraq but one prominent opposition Labour party member stepped down from her post to abstain.
The motion proposed by Prime Minister David Cameron's government was passed by 524 votes to 43, a majority of 481.
Six British Tornado fighter jets based in Cyprus were poised to begin raids on militants within days or even hours.
The vote came after a sometimes heated debate in which lawmakers repeatedly questioned Prime Minister David Cameron over the duration and aims of the operation.
The debate has evoked memories of Britain's role in the deeply unpopular US-led invasion of Iraq in 2003 under then Labour prime minister Tony Blair, which led to the death of 179 British personnel over six years.
Labour's Rushanara Ali, a party spokeswoman on education, said she had resigned from the front bench in order to abstain after the party leadership backed the government's proposal.
"I understand the case that has been made and will not be voting against the motion. But I am unable in conscience to support the motion," she wrote to Labour leader Ed Miliband.
Ali was elected in 2010 in the multi-ethnic constituency of Bethnal Green and Bow in east London, which has a large Muslim population.
"I am not confident that this military action will be effective in the short-term in just targeting the terrorists and not harming innocent civilians," she said.
"Nor can I pretend to have any confidence that there is a credible long-term strategy to build up the capacity of the Iraqi army or that the potential impact on radicalization in the UK has been properly thought through.
"Too many mistakes have been made over the last decade and far too many people in conflict zones have had to pay a high price for misconceived actions by the UK and other countries."