It would be difficult for Britain to say No if Donald Trump asked for assistance in military action against the regime of Syrian President Bashar Assad in response to further chemical attacks, Boris Johnson has said.
And the Foreign Secretary indicated that Prime Minister Theresa May might not seek MPs' approval in a parliamentary vote before British forces were deployed.
Liberal Democrat leader Tim Farron said it was "critical" for ministers to obtain Parliament's consent before taking action, and warned Mrs May not to do so under cover of the election period, when there were no MPs in place.
Labour leader Jeremy Corbyn said the Government should focus on securing a political settlement in Syria, rather than taking unilateral action.
Mr Johnson said there was "no question" that Assad's regime was responsible for the sarin gas attack earlier this month which prompted a US retaliatory cruise missile strike on a Syrian air base.
Asked whether Britain would be ready to join the US in similar action if the chemical assault was repeated, Mr Johnson told BBC Radio 4's Today programme: "I think it would be very difficult for us to say no.
"How exactly we were able to implement that would be for the Government, for the Prime Minister.
"But if the Americans were once again to be forced by the actions of the Assad regime - don't forget, it was Assad who unleashed murder upon his own citizens with weapons that were banned almost 100 years ago - if the Americans choose to act again and they ask us to help, as I say, I think it would be very difficult to say no."
Britain is carrying out air strikes in Syria and Iraq against terrorists of the so-called Islamic State (IS), but has taken no military action against the Assad regime.
MPs backed the action against IS in a December 2015 Commons vote but rejected strikes against the Syrian government in 2013.
Mr Johnson said that the question of whether parliamentary approval would be needed for any military action "needs to be tested".
Pressed on whether the Government would have to bypass Parliament if there was a phone call in the night calling on the UK to support US action, he told LBC radio: "You are putting your finger on the issue."
There was no doubt about Assad's responsibility for the April 4 chemical attack on Khan Sheikhoun in north-western Syria, in which more than 80 people are believed to have died.
"We know Assad did it," the Foreign Secretary said.
"The US responded. On that occasion they did not ask us for assistance.
"If Assad were to do something like that again and if there was a response that was commensurate and appropriate and timely and all the rest of it and the US were to come to us and ask us for support and practical assistance, you would have to work out what the procedures would be but I think it would be very difficult for us as a country given our relationship with the United States for us to say no."
There is no legal requirement on the Government to seek parliamentary approval before ordering military action but it has become convention to consult MPs, except in the event of an emergency, according to the House of Commons library.
Mr Farron said: "Johnson's claims that Theresa May would back further intervention against Assad in Syria by the USA raise serious concerns about the Conservatives' willingness to pander to Donald Trump."
Warning Mrs May against "rushing headlong" into supporting further unilateral action by Mr Trump, Mr Farron said: "Critically, the Government must seek the consent of Parliament.
"Gone are the days where a Prime Minister can take us to war without democratic backing unless in exceptional cases of national security.
"May would be wise not to use the cover of an election to push this through."
Labour leader Jeremy Corbyn said: "We don't need unilateral action, we need to work through the UN, but above all we need to bend ourselves totally to getting a political settlement in Syria and allow the inspectors space to work, allow them to make sure we know who did that terrible chemical weapons attack and also recognise that the inspectors there are already destroying any stocks of chemical weapons that they find."
Downing Street said: "The US have been clear that they are not planning any further strikes, but they could reconsider if the Syrian regime uses chemical weapons again and anything beyond that is hypothetical."
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