Momentum has been accused of attempting to “stitch up” the Labour leadership contest after balloting its members on whether it should back Rebecca Long-Bailey without offering an alternative.

To the fury of senior figures inside the pro-Corbyn campaign group, its 40,000 members were on Tuesday asked to vote “yes or no'' on whether it at as an organisation should support the shadow business secretary’s candidacy.

Momentum, whose founder Jon Lansman is backing Ms Long-Bailey, said it would throw its weight behind her should 50 percent of respondents to the survey agree.

Labour moderates claimed it was “ridiculous” that members had not been asked their opinion on the four other candidates, pointing out that Momentum had been founded to make Labour more democratic.

They were joined by Paul Mason, the former BBC journalist and prominent Labour activist, who likened the move to a survey carried out by a “South American dictatorship”.

It came as backbencher Lisa Nandy threatened to eclipse Ms Long-Bailey as the female front runner, as she secured the endorsement of the National Union of Mineworkers and was tipped to secure another from GMB, one of Labour’s biggest union backers.