Thousands of British Airways cabin crew are to be balloted for fresh strikes in their bitter dispute with the airline, threatening disruption to flights in the new year.
Unite accused the company of continuing with its "vicious war" against the workforce, saying it had "no choice" but to give its members another vote on whether to continue with industrial action.
The move came just hours after shareholders of BA and Spanish carrier Iberia voted in favour of a £5 billion merger of the two airlines.
BA accused Unite of breaking a promise on an agreement last month to end the 14-month dispute, saying the new ballot would create "fresh uncertainty" for customers and damage the interests of thousands of workers at the airline.
Tony Woodley, joint leader of Unite, said the union was making sure its membership records were up-to-date before starting the new strike ballot so it could avoid a repeat of legal action by BA a year ago which led to the union calling off planned strikes over Christmas.
He added that Unite had accepted an invitation from the conciliation service Acas to hold talks with BA next Monday.
Balloting is set to start in the next few weeks, with the result due in early January, so that strikes could be held from mid-January.
Cabin crew have taken 22 days of strike action this year, costing the airline more than £150 million, in a dispute which started over cost savings but developed into a row over staff travel concessions withdrawn from those who took action as well as disciplinary action.
Mr Woodley said: "BA told us it was a business in crisis. They demanded structural change. These changes have been made and this business is now in profit with senior management filling their wallets with the spoils."
A BA spokesman said: "Tony Woodley shook hands with us on an agreement in October and said he would let cabin crew vote on the deal with a recommendation for acceptance. Unite has broken this promise and instead has now chosen to create fresh uncertainty for customers and damage the interests of thousands of its own members within British Airways."