The general secretary of the GMB union, John Edmonds, is to step down a year earlier than expected.
An election to pick his successor is likely before the union's congress next March.
A spokeswoman said the move was for technical reasons so deputy general secretary Steve Pickering could retire at normal retirement age.
The elections for both posts have to be held together, she said.
Mr Edmonds, 58, first led the union in 1986 and was re-elected in 1991 and 1996.
The development sets up an intriguing battle for the leadership of the general union between London regional officer Paul Kenny and northern region officer Kevin Curran.
Reports say Mr Curran may have the tacit support of Prime Minister Tony Blair and Mr Edmonds.
Both candidates are said to regard themselves as centre-left.
There are no candidates yet for the post of deputy general secretary.
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