Britain's biggest union faces being plunged into chaos with both leaders fighting each other in a bitter internal election, The Times has learnt.

Unite, which is led jointly by Derek Simpson and Tony Woodley, is braced for a legal challenge over Mr Simpson's desire to stay on beyond the usual retirement age of 65.

Mr Simpson is said to be considering calling a snap election among members, either for the whole union or possibly for his Amicus side of it, with the intention of staying on for five more years until he is 68. He would face a challenge from Mr Woodley if an election were called for the union's whole membership, triggering a public battle for the control of Unite.

The super-union, with more than two million members, has been riven by deep divisions between its leaders and a clash of cultures since it was formed just over a year ago by the merger of Amicus and the T&G. An election would spark a battle between the Left and Right of the most powerful union in Britain and could prove uncomfortable for Labour. Mr Simpson has been a loyalist to Gordon Brown, launching a stinging attack on David Miliband, Mr Brown's Cabinet rival, last weekend and hiring Charlie Whelan, Mr Brown's former spindoctor, as head of politics at the union.

Mr Woodley led union efforts to seek a left-wing alternative to Mr Brown last year. He also backed Jon Cruddas's unsuccessful campaign to be Labour's deputy leader.

The legal challenge to Unite's leadership is being mounted by Jerry Hicks, a former Amicus executive member. The unions watchdog, the certification officer, is conducting an investigation and has given Unite until Friday to present a defence.

A snap election would help to solve the infighting that has plagued the union since its formation.

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