Britain could join the United States in taking military action against the Syrian regime if it is asked and may bypass a House of Commons vote to authorise it, Boris Johnson has suggested.
The Foreign Secretary said it would be "very difficult to say no" if Donald Trump's administration asks the UK for help in taking action against Bashar Assad's regime in response to another chemical attack.
The US launched a cruise missile strike on a Syrian regime air base this month after Assad allegedly killed scores of people in a sarin gas attack on a rebel-held area in Idlib province.
Asked if MPs would have to approve Britain joining any similar action in the future, Mr Johnson told BBC Radio 4's Today programme: "I think that needs to be tested."
Mr Johnson said Theresa May backed his view that the UK would find it difficult to say no to a request from the US.
He said: "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."
Asked if the PM would have to win a Commons vote before taking action, he replied: "I think that needs to be tested.
"I think it would be very difficult for us to say no."
Asked if the approval of MPs was a necessary pre-condition, he replied: "As I said, 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."