THERESA May has paved the way for British involvement in US-led action against Syria after Donald Trump upped the belligerent rhetoric, telling Russia to “get ready” for military strikes.

The Prime Minister, facing increasing calls for MPs to be given the final say in a Commons vote on any British involvement, made clear that “all the indications” were that the regime of Bashir al-Assad was responsible for the suspected poison gas attack on the Damascus suburb of Douma last weekend.

President Assad has denied any responsibility while Moscow has insisted there is no proof of a chemical attack, suggesting it had been faked by Syrian rebels to draw America into the conflict.

But Mrs May, who will make a Commons statement on Syria when Parliament returns from its Easter recess on Monday, said: "All the indications are that the Syrian regime was responsible and we will be working with our closest allies on how we can ensure that those who are responsible are held to account and how we can prevent and deter the humanitarian catastrophe that comes from the use of chemical weapons in the future.

"The continued use of chemical weapons cannot go unchallenged," she insisted.

Given that the US President had on Monday promised a response within two days, speculation was rising that a military strike could be just hours away.

As Downing Street denied reports the PM was seeking to put a brake on a swift military response, opposition parties stressed the need for a parliamentary vote before the UK became involved.

Jeremy Corbyn said: “Parliament should always be given a say on military action. That’s a case that I’ve made going back many, many years in Parliament.”

The Labour leader described the situation in Syria as very serious and again stressed for the main players to get round the table. “There has to be, now, a demand for a political process to end the war in Syria. We cannot risk an escalation even further than it’s gone already,” he said.

Stewart McDonald for the SNP wrote to Mrs May, making clear his party could not support a strategy that did not seek an end to the civil war in Syria but which simply escalated the violence.

He stressed: “The UK is only actively engaged in Syria because the UK Government carried a motion on December 2 2015 in which it asked Parliament to approve the ‘exclusive’ targeting of Daesh in that country. If the UK Government wishes to expand its involvement, then it would be a scandal if it did so without a full debate and vote in the House of Commons.”

Sir Vince Cable, the Liberal Democrat leader, urged the UK Government to share with Westminster the evidence it had of the Syrian Government’s involvement and that of its Russian backers in last weekend’s suspected chemical attack.

He too called for a parliamentary vote on any UK involvement in military action, saying: “The Government must present the objectives of any proposed action to Parliament. A unilateral response by any country, outside of a wider strategy, without allies is not the way forward. There must be a debate and vote in the House of Commons ahead of any military action.”

Earlier, a war of words broke out between Russia and America.

Alexander Zasypkin, the Russian envoy to Beirut, said: “If there is an American strike, then we...will shoot down the missiles and target the positions from where they were launched. In the past few days, we have seen an escalation towards a significant crisis."

This led Mr Trump to take to social media, tweeting: “Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and ‘smart!’ You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!”

President Assad denounced the US President’s tweet as “reckless” while Moscow insisted it was not interested in engaging with Mr Trump in a Twitter-storm. Vladimir Putin was conspicuous by his silence.

On Tuesday, Russia vetoed a draft US resolution at the United Nations which sought an investigation to determine responsibility for the Douma attack.

Earlier, the Organisation for the Prevention of Chemical Warfare said it was preparing to send a team to the area “shortly”.

The World Health Organisation also demanded “immediate unhindered access” to Douma, saying an estimated 500 patients went to health facilities with “signs and symptoms consistent with exposure to toxic chemicals” after the suspected attack.