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."

Britain is currently carrying out air strikes in Syria and Iraq against so-called Islamic State (IS) terrorists 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 Assad regime in 2013.

After losing the 2013 vote, then-PM David Cameron said: "I strongly believe in the need for a tough response to the use of chemical weapons, but I also believe in respecting the will of this House of Commons.

"It is very clear tonight that, while the House has not passed a motion, it is clear to me that the British Parliament, reflecting the views of the British people, does not want to see British military action.

"I get that, and the Government will act accordingly."

Mr Johnson said there was "no question" that Assad's regime was responsible for the chemical weapons attack.

He told LBC: "We know Assad did it. 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."

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 replied: "You are putting your finger on the issue."