Theresa May’s former co-chief of staff has called on her to resign to stop leading Britain through a "national humiliation".

Nick Timothy, who resigned after writing the manifesto that stripped the Conservatives of their majority in the 2017 election, said his old boss’s mission to deliver Brexit had failed.

He claimed the prime minister’s authority was shot as senior members of her cabinet jostle to replace her.

It’s now beyond time for the prime minister to accept that the game is up, Mr Timothy wrote in the Daily Telegraph.

Her premiership has failed, and her authority is shot.

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.

Mr Timothy said the PM had negotiated a Brexit deal she knew her party opposed and which commanded no Commons majority.

She ran down the clock and refused to seek alternative arrangements to the deal’s most contentious clauses, he added.

As a result she unleashed the Brexit Party, which without dramatic change will consign the Conservatives to opposition.

Mr Timothy said it was not yet clear who should become the new prime minister, but added: That is not the point.

The bruising article comes hours before Mrs May faces a showdown with the 1922 committee executive – the group that represent all Tory MPs.

One of them has told Sky News the prime minister would be asked for a clear timetable or a departure date will be set for her.

Mrs May has said there will be a vote on the Withdrawal Agreement Bill in the first week of June.

That is the law that would enshrine her EU divorce deal brokered with Brussels in November in law.

But following three defeats in the Commons on the deal so far, its passage through parliament is far from guaranteed.

One Tory Brexiteer told Sky News the bill would never pass.

She’s missed the point that she can’t pass a deal, they said.

Somebody else might. She can’t. Nobody trusts her.

(c) Sky News 2019: Theresa May must quit to end ‘national humiliation’, says PM’s ex-adviser