It would be difficult to conceive of a higher stakes gamble by Andy Burnham, mayor of Greater Manchester – the ultimate winner-takes-all move. It is also a huge moment for UK prime minister Keir Starmer as he battles to save his faltering premiership.

Burnham’s plan to return to parliament in Westminster via an extremely risky byelection in the seat of Makerfield near Wigan has shot a bolt of lightning through a looming Labour leadership contest.

If Burnham wins, he may win all. If he loses, his career is effectively over.

As a sitting mayor, Burnham must still get the approval of Labour’s ruling national executive committee (NEC) to run in a byelection in the constituency, whose sitting MP Josh Simons stepped down on Thursday to clear the way. But even that question alone is a test of Starmer’s dwindling strength.

The NEC is meant to be stacked with the prime minister’s allies and it previously blocked Burnham from running in February’s byelection in Gorton & Denton, which had the effect of preventing him from returning to Westminster to challenge Starmer.