Dave Prentis, who recently criticised Jeremy Corbyn, is standing against three other candidates for post of general secretary

The head of Britain’s biggest public sector union is facing a challenge from the left for his position as general secretary.

Dave Prentis, who has been Unison’s general secretary since January 2001, is standing against three other candidates in an election for his position. Voting closed in early December and the results will be announced on Thursday.

Prentis, who backed Jeremy Corbyn in the Labour leadership election, recently broke rank to criticise the new leader, saying that the party must get its act together so it could defend low-paid workers facing the biggest attack on their living standards in a generation.

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General secretaries of trade unions are required by law to be elected at least every five years and Prentis faces a challenge from the union’s head of local government, Heather Wakefield, and branch secretaries John Burgess and Roger Bannister – all of whom are considered to be further to the left than him.

To run for election a candidate must be nominated by either the union’s national executive council, at least two national service group executives, at least two regional councils or by at least 25 Unison branches.

Prentis, who has been nominated by 204 branches, eight regional councils, six service groups and the national executive council, is widely expected to win the contest. Wakefield was nominated by 82 branches, one regional council, and one service group; Burgess was nominated by 62 branches and one regional council; and Bannister was nominated by 25 branches.

There have been calls on the left for all three anti-Prentis candidates to join forces and choose one , to increase their chances of beating the incumbent. The announcement will come as Unison is forced to investigate claims that paid officials attempted to fix the re-election of Prentis.

A Unison official was recently suspended and an inquiry launched when a recording of a meeting – in which paid officials appear to be discussing how to assist Prentis’s election campaign – was published anonymously on a blog. It is against union rules for a paid union official to campaign on behalf of a candidate.

In the recording, leaked online, an official can apparently be heard briefing other members of staff on the progress of Prentis’s election campaign during a meeting of the union’s Greater London regional office on 21 October. “The SWP and the ultra-left in the Labour party are right behind [candidate] John Burgess and have a machine that they will attempt to get a vote out for,” the official can be heard saying.

“This is one of the toughest election that Dave has had to fight and it is very important that Dave is delivered, as I have said before, that not one of the others is capable of running the union, quite frankly,” said the official, who has been suspended pending investigation.

They added: “[If the] opposition gets evidence that people are using Unison resources, which includes staff, there could be complaints, so you just need to be careful and use your discretion and do not use emails as these can be traced so easily.”

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Burgess, who is an associate of the shadow chancellor, John McDonnell, has written to the returning officer for the general secretary election, complaining that the recording “offers clear evidence of a pre-planned and coordinated blatant breaches of the election procedures”.

Burgess called for Electoral Reform Services – who are acting as the returning officers in the union’s general secretary election – to urgently investigate the evidence and said the ballot should be rerun if Prentis is declared the winner on Thursday.

“If in the course of the investigation there is clear evidence of organised abuse of the election procedures on behalf of Dave Prentis, then he must be disqualified from contesting the rerun election,” he wrote.

Unison confirmed that a complaint involving the London region had been sent to Electoral Reform Services. “They are an independent organisation with many years’ experience of scrutinising union and other organisations’ ballots and are now investigating the matter,” the union said.