Britain could launch strikes against the Syrian regime without the backing of the United Nations, according to its own legal advice.
The advice states the legal basis for any such action would be "humanitarian intervention," even if permanent UN members Russia and China block it.
UK intelligence chiefs have told the prime minister it is "highly likely" the Syrian government was responsible for a chemical attack last week.
British Parliament is now debating the issue.
Opening the debate as the Commons was recalled, Prime Minister David Cameron said there was "no 100 percent certainty about who is responsible" for the attack in Syria, but told members of parliament "you have to make a judgment."
He insisted he was convinced by evidence suggesting the regime was responsible.
President Barack Obama has said he has not yet decided on a plan for retaliatory action against Syria. Other nations are also considering the next move.