The union representing British Airways BAY.L cabin crew said on Tuesday it planned to ballot members again on taking strike action after saying a recent vote was potentially invalid.

Cabin crew voted last month to hold further strikes in a long-running dispute which has already cost the airline some 150 million pounds.

However, BA argued that the ballot, the third held by staff backing industrial action, was unlawful. That led the Electoral Reform Society, the watchdog responsible for overseeing the vote, to qualify its official report, the union said.

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 the union's general secretary Len McCluskey.

He said the union, which represents about 90 percent of the airline's cabin crew, had had no contact from BA bosses since last month's ballot and they would now hold another vote on taking industrial action.

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, McCluskey said.

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.

The dispute began over proposed cuts to pay and staffing levels but has now broadened into a fight over issues related to strikes held last year.

The result of the most recent ballot was announced on the day BA completed its merger with Spanish airline Iberia to form a new holding company, International Consolidated Airlines Group IAG.L (ICAG.L). (Reporting by Michael Holden; Editing by Will Waterman)