Members of the non-TUC affiliated Cabin Crew `89 voted overwhelmingly to merge with the AEEU, which until recently had no stewards and stewardesses in membership.
The amalgamation means that the engineering union will be at loggerheads with the Transport and General Workers' Union, the traditional organisation for cabin crew and the largest union at BA.
Leaders of the transport union are already angry with the AEEU for winning recognition at BA's cut-price airline GO, having been selected by management following a "beauty contest" between unions.
The TGWU may now call for the engineering union to be expelled from the TUC for trespassing on its territory.
Cabin crew `89 was set up by members of the transport union who said they wanted a more "moderate" organisation to represent them and one which concentrated only on stewards and stewardesses. The non-TUC union claims a membership of 5,500 who voted by a margin of 93 per cent for a merger with the Federation of Professional Associations, a wing of the AEEU.
An engineering union official said: "We are in discussions with the TUC over the future of Cabin Crew `89, but it would be inappropriate for me to comment further at this stage."
Bill Morris, the TGWU general secretary, said Cabin Crew' `89 was a "breakaway union" and that therefore a TUC affiliate such as the engineering union could not merge with it.
"It is against the rules of the TUC to go ahead with such a merger and it would harm the cohesiveness of the trade union movement," Mr Morris said.
He added that it was now the function of the TUC's ruling general council to sort out the problem. He said he expected the AEEU to abide by the rules.
The AEEU recently angered print unions when it agreed to make a presentation to the Western Mail and Echo newspaper with a view to represent all employees. The MSF union has also submitted proposals to Trinity, the company owning the papers - although under pressure from the TUC, it has also invited the GPMU print union and the NUJ.