Jeremy Corbyn could face leadership challenge if Labour underperform in May elections, admits Momentum chief

Party activist Jon Lansman told supporters that Sadiq Khan winning the London Mayoral elections was 'absolutely crucial'

Jeremy Corbyn will have a fight on his hands if Labour does not do well in the first major test of his leadership (Getty Images)

Jeremy Corbyn could face a leadership challenge if Labour does poorly in the May elections, the head of activist group Momentum has admitted.

Party activist Jon Lansman told supporters they had to be “in a position to be able to run another leadership campaign” if they failed to capture the London Mayoral job and several key council seats at the local elections.

Speaking in Haringey, north London, at a branch meeting of the campaign group - which was launched to support Mr Corbyn after he won the leadership race in September last year - Mr Lansman said:"I’m in no doubt that after May we could face a leadership election.”

He said getting Sadiq Khan elected as London Mayor was “absolutely crucial”, the Socialist Worker reports.

The long time left-winger said: "We’ve had a very rough time—Jeremy and John (McDonnell) particularly. You hear about Momentum bullying.

"What happens in the Parliamentary Labour Party every Monday is bullying by a small section of the PLP. What it is designed to do is grind Jeremy."

But he said they were not "under any illusions" that they were going to do well in the Scottish Holyrood elections taking place at the same time.

"We are going to lose more seats in Scotland", he said.

It comes as sources close to Mr Corbyn predicted at the start of the year that it would be “the beginning of the end” for the leader if Labour failed to win at least 35 per cent of the vote in the elections.

Mr Corbyn was given a reprieve when the party performed better than expected in the Oldham West and Royton by-election in December.

New MP Jim McMahon secured a 10,722 vote majority in the by-election in December which triggered by the death of veteran Labour left-winger Michael Meacher.