Cabin crew at British Airways are to take a new ballot on strike action after the airline said a previous ballot was illegal.Staff voted in favour of strike action last week, but BA invoked a ruling that staff can be dismissed if they go on strike for the same reason more than three months after they first went on strike. BA pointed out that the union representing the workers, Unite, in planning a new action had cited the same grievances that prompted a strike last year."Unite therefore cannot call industrial action based on this ballot, since such a move would expose our members to sanctions by a bullying employer," the union said in a statement.It added that it is "immediately setting in hand preparations for a further industrial action ballot on the issues arising from management's breach of agreements and policies of industrial intimidation."The dispute has been running for almost two years and cost BA, which is now part of International Consolidated Airlines since its merger with Spain's Iberia, £150m. An agreement looked to be near a few months ago, but that deal fell apart.Unite wants BA to reinstate travel concessions for striking staff and agree to an external review of all disciplinary cases against members.A statement on BA's website today said that "Unite has failed to conduct the process properly and has announced that it cannot call strike action on the basis of this ballot."