Britain’s biggest trade union could be heading for a dramatic break-up, destroying what was hailed as a big step forward for the labour movement.

The Times has learnt that the T&G section of Unite has sought legal advice about pulling out of its troubled merger with the Amicus union, amid tensions between its joint leaders.

Unite has been hampered by friction between Tony Woodley, from the T&G, and Derek Simpson, from Amicus, who share power as joint general secretaries.

Their relationship has been marked by rivalry and suspicion. Mr Simpson failed to attend the launch of Unite last year after Mr Woodley was interviewed about it that morning on BBC Radio 4’s Today. Insiders say they can barely be in the same room together.

The T&G side is also very unhappy with Amicus’s finances after it went £3.6 million into the red in the past six months. Amicus says it is paying for some operations that serve both unions, such as communications.

The confirmation of the severity of the problems at Unite comes as Mr Simpson prepares to stand for re-election to bolster his own position. He will submit himself to an election among members of his Amicus side of the union after a legal challenge to his plans to stay on beyond retirement age.

As revealed in The Times last month, Mr Simpson is seeking a fresh mandate in the face of the legal challenge, which is being considered by the unions’ watchdog, the Certification Officer.

If Unite, which has 2.1 million members and gives £2 million a year to the Labour Party, were to split up it would be a big blow to the union movement, which is increasingly using mergers to offset declining membership and job losses in traditionally unionised areas such as manufacturing.

Mr Simpson will face a serious internal challenge for his job from Laurence Faircloth, a regional official. A third candidate will be Jerry Hicks, a former Rolls-Royce official who is mounting the legal challenge to Mr Simpson’s tenure. The election will be held early next year.

Mr Simpson had intended to quit as joint general secretary of Unite in late 2010, when he is 66.

Mr Woodley plans to step down early the following year and hand over to a single successor for the merged union, who is due to be elected next year.

Mr Simpson has been an ardent supporter of Gordon Brown, giving a job to Charlie Whelan, his former spin doctor, and if he were toppled it would be a blow to the Prime Minister.

An emergency meeting of Unite’s ruling executive yesterday voted to approve his plan to hold the election. Significantly, it also suspended for six months the new rule book for the merged union. This means the two sides remain individual organisations. The full merger was supposed to have been sealed, with the new rule book, on November 1.

Although formal plans to merge were laid out in May last year, the two sides are still at odds over a financial structure. Finance is a key issue for the T&G, where officials are also unhappy with the spending of Mr Simpson. He has been criticised for living in a house paid for by the union and for other costs such as twice taking a helicopter to the Glastonbury Festival.

Mr Faircloth, 56, Unite’s regional secretary for the southwest, said he was standing against Mr Simpson because he did not believe enough was being done to push through the merger. The election will enable the new joint general secretary to serve only for a year, finishing in 2010. Unite officials declined to comment.

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