Theresa May believes it would be difficult to say no if Donald Trump asked the UK for help to bomb the regime of Syrian President Bashar Assad, the Foreign Secretary has said.
Boris Johnson also suggested that the Prime Minister might not seek the approval of MPs before British forces took part in a strike.
The SNP’s Foreign Affairs spokesman Alex Salmond accused the Conservative Government of having a "'mini-me'" policy approach to the controversial American President.
Opposition parties also urged Mrs May to wait consult parliament before taking any action.
Mr Johnson said that there was no question that Assad's regime was behind a devastating sarin gas attack earlier this month.
In response, Mr Trump shocked the world with a retaliatory missile strike on a Syrian air base.
Asked if Britain would be join the US in similar action if there another chemical assault, Mr Johnson told the BBC’s Today programme: "I think it will be very difficult if the United States has a proposal to have some sort of action in response to a chemical weapons attack, and if they come to us and ask for our support, whether it's with submarine-based cruise missiles in the (Mediterranean), or whatever it happens to be, as was the case back in 2013, it would be in my view, and I know this is also the view of the Prime Minister, it would be very difficult for us to say no."
Britain is currently carrying out air strikes in Syria and Iraq against Islamic State (IS) terrorists.
But the UK has not been involved in action against the Assad regime.
In 2013 MPs against strikes on the Syrian government.
But two years later they backed action against IS.
Mr Johnson said that the question of whether parliamentary approval would be needed for any military action "needs to be tested".
There is no legal requirement on the Government to seek parliamentary approval before ordering military action.
Labour leader Jeremy Corbyn said Mrs May should consult other parties if she received a request for military support while MPs are around the country campaigning in the General Election.
Lib Dem leader 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."
A Downing Street spokesman 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."
Source close to the Foreign Secretary said that there had been no change in position.
Mr Johnson also called for "calm, clear heads" in the stand-off with North Korea, saying that military action against Pyongyang was "not the way forward".
Mr Trump has recently ramped up pressure on the North Korean regime.
On Wednesday he took the unusual step of inviting every US senator to the White House for a classified security briefing on the threat from the country's missile test programme.
"The situation in North Korea has changed very substantially over the last few years. What people thought was an almost comical question - the North Korean nuclear threat - has become very real and very dangerous indeed," said Mr Johnson.
"We need to address it. I think the White House are entirely right in escalating the seriousness of this question. All North Korea's neighbours feel this threat very intensely.
"To what extent is there a military solution? I have to say I am very, very sceptical and so are most of the experts that I've talked to.
"I think the military options are not good. The best way forward - and I think this is what the White House wants to pursue - is to keep a calm, clear head and to work particularly with Beijing to try to bring pressure on Pyongyang, try to get them to see that they could have a great economic future if they could agree not to be so threatening, if they could agree a freeze or to denuclearise."
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