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A bid by Jeremy Corbyn’s supporters to gain control of the London Labour Party suffered a major blow after a Left-winger failed to be elected to a key post.

Barrister Ellie Reeves, a centrist candidate, saw off a challenge from lawyer Niamh O’Brady to become vice-chairwoman of the London Labour Party regional board.

Ms O’Brady had welcomed support from a local Labour party in the capital as a “step towards a board supporting [Jeremy] Corbyn”.

Before the vote, Ms Reeves believed that the Unison union was going to back her but claimed its support was withdrawn hours before the ballot after the Unite union, led by Len McCluskey, allegedly intervened.

The maternity rights campaigner, who is married to Labour MP John Cryer and is the sister of former shadow Cabinet member Rachel Reeves, was stunned by her win on Saturday.

Nearly 85 per cent of the constituency Labour parties in London backed her and Unison abstained. Afterwards, Ms Reeves said: “It was a victory, not just for me but for ordinary members. From what I can gather, Unite tried to block my election. But the members fought back against that and won.”

However, Unite denied that union bosses, including Mr McCluskey, had had any involvement in the vote or that any pressure was applied on Unison.

A spokesman said: “In Unite, these matters are decided in the London region and in full consultation with sister unions.”

Ms Reeves lost her seat on Labour’s National Executive Committee this year and her win, by 55 per cent to 44 per cent, of the London post was welcomed by centrist Labour MPs.

Labour’s deputy leader Tom Watson, who has clashed with Jeremy Corbyn over his leadership, tweeted: “Congratulations Ellie. An excellent result for a very worthy candidate.”

Jim Fitzpatrick, MP for Poplar and Limehouse, said: “It demonstrates that Unite do not control the London Labour Party.”

The regional board has influence over which wing of the party has sway in the capital because it has a say in resolving disputes over council candidates.