Roger Lyons, veteran joint leader of Britain's biggest private sector union, is seeking to extend his term in office to balance the influence of his newly elected left-wing colleague Derek Simpson.

Mr Lyons, who had been due to step down from his role at Amicus in 2004, at the same time as Mr Simpson's defeated opponent Sir Ken Jackson, is believed to be considering staying at least another year.

The move follows the left-winger's successful leadership challenge and comes as Amicus prepares for sweeping staff changes later this month in advance of a move to new headquarters in central London.

In part, it reflects concern in Downing Street and in Mr Lyons's part of the union, formed by this year's merger of the MSF, the general technical union, and the AEEU, engineering union, that the upheaval caused by Mr Simpson's election needs further joint management.

Amicus is made up of two unions that have maintained fierce loyalty to Tony Blair's government. Mr Simpson, a former communist elected on a wave of grassroots dissatisfaction, may signal a change of direction.

In theory, he will become sole leader of the union after Mr Lyons steps down. But, with Amicus also in talks over a possible merger with Unifi, the banking union, officials on the MSF executive are keen that Mr Lyons should stay longer. Mr Lyons's contract allows him to continue until retirement age in 2007, two years before Mr Simpson would have to step down. He will take over as president of the Trades Union Congress next year, but could return to his Amicus role full-time the year after, staying until 2005.