British Airways cabin crew are to be re-balloted for strikes in their long-running dispute following a "legal blitz" by the airline.
Unite said BA had informed the Electoral Reform Society (ERS) that a recent ballot on industrial action was unlawful and that any action taken would be unprotected.
Even though this was "merely an assertion" by BA, the ERS issued a qualification in its report on the ballot, said Unite.
"Unite therefore cannot call industrial action based on this ballot, since such a move would expose our members to sanctions by a bullying employer," said general secretary Len McCluskey.
Unite accused BA of launching a "legal blitz" against the independent scrutineer used to conduct the recent ballot.
Mr McCluskey said: "This doubtless reflects BA's decision to place its chief legal officer in charge of employee relations. While not addressing the union directly, BA advised the ERS that the ballot was unlawful and that any industrial action taken on the basis of it would be unprotected.
"If BA management believes that it can secure industrial harmony by these methods it is living in a fool's paradise. Only negotiation, not litigation or intimidation, can place industrial relations on a sound footing and start to heal the wounds caused by this dispute.
"Failing a willingness by BA management to take its employment relations seriously and start negotiating, we would anticipate giving the company formal notice of this fresh ballot within the next 10 days or so.
"BA management needs to understand that it will never break the spirit of cabin crew, and that customer uncertainty and confusion will continue until it starts listening to its staff."