Theresa May's former chief adviser has told the prime minister "the game is up", and to stand aside in order to end the "national humiliation" over Brexit.

The scathing remarks come ahead of the prime minister's crucial showdown with the executive of the 1922 Committee in Westminster, amid calls for her to specify a timeline to depart Downing Street.

Telling Ms May her premiership had "failed", Nick Timothy, her former chief of staff in Downing Street before the snap general election, said: "Prime minister, I am sorry to say, must do her duty and stand aside."

In a column for the Daily Telegraph, Mr Timothy wrote: "In the local elections earlier this month, she lost almost 1,300 councillors. In the farcical European elections next week, she will win as few as one in 10 votes.

"Now the government wants to introduce the withdrawal agreement bill. So ugly are its contents, it remains unpublished.

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"But when it is finally unveiled, we will see, in black and white, legislation that subordinates a supposedly independent Britain to European laws and the jurisdiction of the European Court of Justice."

He added the "only conceivable" way of the legislation succeeding would be if Labour allowed it to pass - a prospect that looks highly unlikely in the current circumstances.

Mr Timothy continued: "The prime minister's Brexit plan has failed, and she has no alternative. Every day wasted from here makes life harder for whoever leads Britain into the future.

"We need to end this national humiliation, deliver Brexit and save the Tories. The prime minister, I am sorry to say, must do her duty and stand aside."

Referring to a vote of the Tory grassroots of confidence in the prime minister next month, he added: "In June, the Conservative voluntary party will declare no confidence in her as its leader."

Ahead of today's showdown, Sir Geoffrey Clifton-Brown, a Tory MP and treasurer of the 1922 Committee, said: "It would be infinitely preferable if she set a date rather than us force her out.