Theresa May has summoned ministers to an emergency cabinet meeting to discuss the UK’s response to the suspected chemical weapons attack in Syria.
Ministers are expected to back her call to join military action threatened by the United States and its allies.
Sources say the PM is prepared to take action against the Assad regime without first seeking Parliamentary consent.
But there have been calls from opposition parties and some Tories for MPs to get a vote beforehand.
The allies want to prevent a repeat of an apparent chemical attack in the formerly rebel-held town of Douma.
Mrs May has said “all the indications” are that the Syrian regime of president Bashar al-Assad, which denies mounting a chemical attack, was responsible.
Senior figures from Russia, which provides military support to the Syrian regime, have warned of a Russian response to a US attack.
BBC political correspondent Iain Watson said Mrs May appeared to have made up her mind and that it was “a question of when, not if” there will be military action.
If the cabinet approves UK involvement, that would open the way for British forces to join an operation against Syrian targets that US President Donald Trump has said in a tweet “will be coming”.
During a briefing on Wednesday, however, White House spokeswoman Sarah Sanders appeared to row back on President Trump’s warning to Russia that it should “get ready” for missile strikes against its ally.
She told reporters that the president had “a number of options at his disposal and a number remain on the table”, but added: “We haven’t laid out any specific actions we plan to take.”
MPs are due to return to Westminster from the Easter recess on Monday.
Labour leader Jeremy Corbyn has said Parliament must be consulted before military action is taken, and has warned of triggering a “hot war between US and Russia over skies of Syria”.
Brexit Secretary David Davis, one of the MPs to oppose military action against President Assad when it was rejected by the Commons in 2013, said he was assured that evidence and intelligence, as well as a “proper plan”, would be provided this time.
The SNP’s Westminster leader, Ian Blackford, told the BBC a Parliamentary debate should take place before – rather than after – military action has taken place because Mrs May does not have a majority in the House of Commons.
Conservative MP Zac Goldsmith also called for Parliament to have its say before anything is agreed.
But Conservative MP Tom Tugendhat, who chairs the Commons Foreign Affairs Committee, told BBC Radio 4’s Today programme that while “politically it may make sense”, Mrs May did not need to ask for a vote.
He added that a “very targeted operation” at Syrian chemical weapons stocks need not trigger a conflict with Russia.
By BBC diplomatic correspondent James Landale
Slowly but surely the ground is being prepared for war.
Members of the cabinet will return to London later to discuss the government’s response to the suspected chemical attack on Douma.
Theresa May will set out the evidence that she believes shows the Syrian government was responsible, and she will set out the military options being considered with US and French allies.
Key questions will be what British forces might be used against which targets, what will be done to avoid escalating the conflict, and what is the overall strategic aim.
The Ministry of Defence refused to comment on reports that a submarine had been dispatched to the Mediterranean – but it would be surprising if this had not been done.
The cabinet is expected to back the prime minister. But she seems willing to act without the express support of MPs who don’t return to Westminster until next week. This will anger some – like the Labour leader Jeremy Corbyn – who believe Parliament should be consulted first.
Lib Dem leader Sir Vince Cable did not rule out backing military action but he said Parliament would have to give its approval with conditions.
But Conservative MP Chris Philp told the BBC’s Newsnight that although a vote was “desirable”, events can move too fast and it was not a necessity.
Mr Philp also said that, even if there was a vote, a number of Labour backbenchers had already expressed their willingness to side with the government and back military action.
“I wouldn’t make any assumptions about the vote,” he added.
Medical sources say dozens of people were killed, including children, during the alleged toxic bombing of the formerly rebel-held town of Douma, in the Eastern Ghouta region.