Merger plans to create Britain’s biggest union will go ahead in spite of a decision to pull out by one of the potential partners, union leaders said on Wednesday night.

Delegates at the GMB annual conference in Blackpool voted overwhelmingly on Wednesday to reject plans to create a new super-union by merging with the larger T&G and Amicus unions. The combined organisations would have had a membership of more than 2m.

Paul Kenny, GMB general secretary, said that after consulting regions and branches, his union had found “no appetite for a three-way merger”.

Tony Woodley, T&G general secretary, and Derek Simpson, general secretary of Amicus, said they were disappointed by the GMB’s decision. They insisted that the two remaining unions still planned to merge.

A combination of Amicus and the T&G would still create Britain’s biggest trade union with between 1.6m and 1.8m members, making it bigger than Unison, the largest public sector union which has about 1.3m members. The GMB has 600,000 members.

In a joint statement, following the GMB vote, the T&G and Amicus said: “Obviously we are disappointed at this decision, which we regard as missed opportunity for the GMB. However, we fully respect the GMB’s democratic decision and will continue to work closely with our sister union for the benefit of all our members.

“The T&G and Amicus will now proceed with our planned creation of the new union.” The collapse in total trade union membership, from a peak of 13.2m in 1979 to just under 7m, has prompted a series of mergers as organisations have sought to underpin their finances and maintain influence.

Unison was created by merging Nupe, Nalgo and Cohse, the local government and health unions, while Amicus was formed in 2002 by a merger between the AEEU and MSF engineering, electrical and manufacturing and finance unions.

The emergence of large unions has raised concerns about their potential influence over the Labour party and the rest of the trade union movement.

The T&G, Amicus, GMB and Unison combined at last year’s Labour party conference to force through a motion calling for the repeal of laws banning secondary picketing.

The resolution was proposed after British Airways baggage handlers staged a wildcat strike last year in support of more than 600 sacked Gate Gourmet catering workers.

Ministers, however, said they would ignore the conference decision and have called for a review of the party rule book to dilute the influence of large trade union bloc votes.

The GMB, T&G and Amicus between them command more than a quarter of Labour party conference votes and about 40 per cent of votes at the Trades Union Congress. The power at the GMB, however, rests with its powerful regional network while Amicus and T&G have more centralised command structures.

Fears of GMB regional leaders that they would lose much of their autonomy was behind Wednesday’s decision to pull out of the merger.