Boris Johnson’s threat to withhold Britain’s £39 billion divorce payment to get a better Brexit deal would be “unacceptable” and almost certainly illegal, senior figures in Brussels have said.

The former foreign secretary used his first interview of the Tory leadership campaign to warn that he would “retain” the money until he got a better deal with the EU than the one negotiated by Theresa May.

He added that the money would be a “great lubricant” to persuade member states to reopen talks on the controversial Irish backstop and agree the terms of a long-term trade deal.

“Our friends and partners need to understand that the money is going to be retained until such time as we have greater clarity about the way forward,” he