The head of the trade union Unite has warned British Airways that it plans to move towards industrial action against the airline "with immediate effect".
Unite and BA have been at loggerheads for several weeks. In April, BA owner IAG warned it could cut up to 12,000 jobs due to the impact of coronavirus.
Staff were warned that if agreement was not reached, they would be handed their notice and re-hired on new contracts.
BA said it was disappointed by Unite's criticism.
Many airlines are struggling to survive as the coronavirus pandemic has severely disrupted global travel.
The plunge in travel will drive airline losses of more than $84bn (£66bn) this year, the International Air Transport Association has warned. It said last month that 2020 revenues would drop to $419bn, down 50% from 2019.
'Fire and rehire'
For many long-serving staff, BA's plan would involve significant pay cuts, as well as changes to terms and conditions.
Unite - which represents thousands of BA employees including cabin crew, engineers and maintenance staff - has accused the airline of operating a 'fire and rehire policy'.
In a letter to BA's chief executive Alex Cruz, seen by the BBC, Unite general secretary Len McCluskey accuses Mr Cruz of "arrogance" in his dealings with the union.
Mr McCluskey says: "You have now published a timetable to fire and rehire thousands of your workforce on 7 August.
"We will work every hour between now and then to convince you not to do so.
"You can take this letter as our commitment to do that. However, you can also take this as an intention to defend our members by moving towards industrial action with immediate effect".
'Save more jobs'
British Airways said the lockdown and halt in travel "is the biggest challenge the airline and our industry has ever faced".
"It is disappointing that a company doing everything it can to save jobs is being singled out by Unite for national criticism, when jobs are being lost across the country in every industry," it said in a statement responding to Unite's letter.
The new contracts make BA competitive with lower-cost airlines, it said. It insists 40% of cabin crew would get a pay rise, while crew suffering cuts would see their basic pay drop by 20%, although they also stand to lose shift pay, meaning drops of around 40% are possible.
"If staff accept the changes to the way they work or their terms and conditions, we expect to be able to save more jobs."
BA may have reached a cost-saving deal with its pilots, but the confrontation with other employees shows no sign of easing.
From cabin crew to engineers and office staff, there is deep concern about the effect BA's plans will have on livelihoods and families.
But there's also anger at the way in which the company has approached the issue - particularly its plans to dismiss staff and rehire them on new contracts.
BA insists it has no choice - and has consistently said it wants to engage with its workers in its attempts to cut costs. But Unite insiders claim their suggestions have been dismissed out of hand.
The clear breakdown in trust between BA and its workforce - and the increasingly strident language from the union - seem to make an amicable agreement an unlikely prospect.