Senior Unite official Jennie Formby is among five people on the long list to be Labour's next general secretary, The Independent understands.
The list is expected to be reduced to three candidates on Thursday. These will be interviewed by the party's National Executive Committee (NEC) next Tuesday and one appointed.
Ms Formby, previously Unite's political secretary and now the union's regional secretary for the south east, is a close ally of Jeremy Corbyn and is believed to have the support of his team.
Her main rival, Momentum founder and chairman Jon Lansman, pulled out of the race last week.
The general secretary is Labour's most senior employee and oversees operations at the party's Southside headquarters in Victoria. The incumbent, Iain McNicol, stepped down last month, reportedly after being told by members of Mr Corbyn's team that he was being removed.
Mr McNicol has had a strained relationship with the Labour leader's office after he insisted that Mr Corbyn should be forced to re-seek nominations from MPs during the 2016 leadership contest.
The other four candidates on the long list to replace him are Christine Blower, Paul Hilder, David Sayer and Byron Taylor.
Ms Blower was formerly the general secretary of the National Union of Teachers.
Mr Hilder is an entrepreneur who previously worked for Oxfam. He founded the Crowdpac website along with former David Cameron adviser Steve Hilton, and also worked on Bernie Sanders' campaign to become US president. He stood against Mr McNicol for the position of general secretary in 2011 and is now involved with the Momentum group.
Mr Taylor is a former trade union officer and leader of the Labour group on Basildon Council. He stood as the party's candidate in South Basildon and East Thurrock at the last general election.
Less is known about Mr Sayer but he is believed to work for Channel 4.
