Strike action is being threatened by workers at Iberia in protest against restructuring which will lead to 4,500 job losses.

Spanish pilots’ union Sepla has accused British Airways and Iberia owner International Airlines Group (IAG) of a “flagrant disregard” of Spanish interests.

It has threatened to stage legal challenges over the merger agreement, which it claims was stacked in BA’s favour.

“It’s not fair and it’s not the way things were supposed to be. The merger was supposed to bring growth for both companies and benefits for workers of both airlines,” Sepla head Justo Peral told the Daily Telegraph.

“We want to break the merger and go it alone again, that’s the only way we will survive. We will not stand by and see Iberia dismantled by the British.”

The war or words broke out ahead of meetings between Iberia management and unions this week to start negotiating the restructure which calls for a cut in pay and trimming the Iberia network by 15% in addition to the job cuts

Peral reportedly argued that Iberia assets had been used to plug a deficit in the BA pension scheme, estimated at £3.7bn at the time of the merger in 2011, and that Iberia workers would be contributing to BA pensions until 2026, while a quarter of its workforce would be forced to join the ranks of Spain’s five million unemployed.

Iberia management retaliated, accusing Sepla of mounting a propaganda campaign of “repeated lies” and claiming a complete overhaul was necessary to make the 85-year-old airline competitive.

While BA made an operating profit of €286 million in the first nine months of this year, IAG profits were virtually wiped out by Iberia’s operating loss of €262 million.

The Sanish carrier’s chief executive Rafael Sánchez-Lozano said: “Iberia is in a fight for survival. It is unprofitable in all markets. Unless we take radical action to introduce permanent structural change, the future for the airline is bleak.”

The company argued that while BA had undergone a period of readjustment, Iberia had yet to do so and that Iberia pilots, for example, earned on average twice the salary of BA counterparts.

New figures showed Iberia passenger numbers have fallen by almost 50% since Madrid’s Terminal 4 was inaugurated in October 2006.