LONDON (Reuters) – Former foreign secretary Boris Johnson said on Sunday that Prime Minister Theresa May should publish the legal advice she was given on her Brexit deal, joining calls by the Labour Party.

In his weekly column in the Telegraph newspaper, Johnson, who quit his role in protest at May’s Brexit strategy, said the advice would show that a so-called backstop arrangement for Northern Ireland was “a great steel trap that is about to clamp its jaws around our hind limbs and prevent our escape”.

“Yes, of course the public is entitled to see the legal advice from the attorney general to the prime minister,” he wrote. “It is outrageous that the public should be prevented from knowing the full legal implications of this appalling deal – when it is their rights, their freedoms, their hard-won suffrage, that are about to be bartered away.”

May is battling to prevent an exit deal that she has painstakingly negotiated with the European Union being voted down in parliament on Dec. 11.