Speaking on BBC Radio 4’s Today programme, Mr Johnson said: “I think the argument is about how to get out of the backstop, and ensure that the UK is not locked in that prison of the customs union.
“I think that some of the ideas that the Prime Minister has mentioned in the House of Commons do seem sensible. I think you would need to have a time limit.
“I think that I would want to look very carefully at what is being proposed, and it would have to give the United Kingdom a UK-sized exit from the backstop.
“We would have to be able to get out by a certain time and we would have to be able to get out of our own volition. Otherwise your locked in arrangements that greatly suit the EU, that enable them to run our trade policy, meaning that we have not delivered on the result of the referendum.
“Worse than that, we have not secured any of the economic benefits of Brexit.”
Asked if he would accept a legal codicil or clarifications making clear that the UK cannot be trapped in the arrangement indefinitely, Mr Johnson said it would not “be good enough”, adding that officials in Downing Street were of the same opinion.