As reported by Censor.NET citing BBC, the foreign secretary said MPs would not necessarily have a vote on any proposed joint action.
He claimed the Assad regime had "unleashed murder upon his own citizens with weapons that were banned almost 100 years ago."
Johnson says it would be very difficult for the U.K. to refuse the U.S. if it asked for support in another military strike on Syria.
"I think it would be very difficult if the U.S. 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 Med... in my view - and I know it's also the view of the prime minister - it would be difficult for us to say 'no'."
Asked if the Commons would need to be consulted ahead of any military strike, Mr Johnson commented: "I think that needs to be tested."
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