Britain’s former Brexit secretary is urging members of Prime Minister Theresa May’s cabinet to rebel against her proposed deal with the European Union over the terms of Britain’s departure from the bloc.

David Davis wrote in the Sunday Times that May’s plans for some continued ties with the EU under her Chequers plan is “completely unacceptable” and must be stopped. The fellow Conservative Party member said the time has come for ministers to shoot down May’s plan.

“It is time for the cabinet to exert their collective authority,” he said. “This week the authority of our constitution is on the line.”

May is struggling to build a consensus behind her Brexit plans ahead of a cabinet meeting Tuesday that will be followed by an EU summit Wednesday in Brussels.

If Davis’ call for a rebellion is effective, the cabinet meeting Tuesday would be a likely place for opposition to surface.

Davis and former Foreign Secretary Boris Johnson resigned from the cabinet this summer to protest May’s Brexit blueprint. Both have become vocal opponents of her plan, calling it a betrayal of the Brexit vote that would leave Britain in a weakened position.

May also faces obstacles from the Democratic Unionist Party in Northern Ireland, which has played a crucial role in propping up her minority government in Parliament.

DUP leader Arlene Foster remains opposed to any Brexit plan that would require checks on goods traveling between Northern Ireland and Britain, as some EU leaders have suggested as part of a “backstop” plan.

The Chequers plan has also been questioned by some opposition Labour Party lawmakers, further complicating the prime minister’s hopes of winning parliamentary backing for any Brexit deal she reaches with EU officials.