Prime Minister Theresa May has been "undermined" and risks signing up to the "very worst" Brexit deal for Britain, her former chief of staff has said.
Nick Timothy has written in the Daily Telegraph that "the time for playing nice and being exploited is over" and that Britain must "toughen up".
The intervention of Mr Timothy, who left his position in the wake of last year's snap general election, comes as Mrs May heads to Brussels for a European Union summit.
He wrote: "Britain's negotiating position is far from hopeless.
"Member states are divided about what the Brexit deal should say about security and trade. Almost all, in their different ways, want a sensible agreement.
"But the time for playing nice and being exploited is over. Of course we must negotiate in good faith. And we will need to compromise.
"The time for sincere co-operation with a partner that does not want to sincerely co-operate is over: we must toughen up."