The question everyone in Westminster has been asking since Andy Burnham announced he was standing in the Makerfield by-election: what happens if and when he wins? How long until he challenges the prime minister?

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No one is under the illusion that the summit of Burnham’s ambitions is to be Makerfield’s representative in Parliament.

He gave up the job he has held for nine years for one reason only: to become Prime Minister.

Indeed, when asked at a televised hustings, the Mayor of Greater Manchester said: "I think Wes Streeting seems to have launched a leadership contest, so if that is running I would seek to join it."

So: now he has won. What happens next?

He will be sworn in as an MP in Westminster on Monday afternoon.

He was disqualified from being Greater Manchester mayor as soon as he was elected to Parliament. At that point, a “vacancy arose” for the mayoralty and the returning officer has to hold a by-election within 35 days. July 30 has been identified as the date the contest to replace Burnham will be held.

Some in Number Ten hope to delay any challenge to Sir Keir until that by-election is out of the........

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