A regional officer of Unite is to announce that he will challenge Len McCluskey for the job of leading the union.
Gerard Coyne, who is regional secretary for the West Midlands, will give a press conference from the roof of the landmark Fort Dunlop building in Birmingham.
He has been a union official for more than 20 years.
Mr McCluskey announced last week that he will stand for general secretary again next spring, bringing forward the election by a year. He has twice been elected leader of the country's biggest trade union.
Mr McCluskey is a strong supporter of Labour leader Jeremy Corbyn and will be in post until the next general election if he wins.