The Unite union has said it may ballot British Airways cabin crews for further strike action, raising the chance of walkouts in the summer school holidays.
The warning came on the second day of the latest five-day strike by Unite members, with another walkout of the same duration due to start on 5 June.
Unite joint secretary Tony Woodley said a new vote may come within two weeks.
BA has yet to comment on the threat of additional industrial action.
If Unite cabin crew members were to vote for further strikes, it would likely mean walkouts starting in July, unless it and BA can come to an agreement in the meantime.
Both sides have said they would like fresh talks, but as yet none have been announced.
The long-running dispute between Unite's cabin crew workers and BA centres on jobs, pay and working conditions.
Unite is also demanding that BA reinstates the travel perks of striking workers, and cancels disciplinary proceedings against others.
The union has offered to call off the strikes if the travel concessions are put back in place, ahead of agreeing a final deal.
BA says it has offered to reinstate travel concessions, but only as part of a final agreement.
Mr Woodley made the warning of further strike action at Unite's annual conference in Manchester.
He said its members were standing up to BA's "bullying".
"We all know there is a deal to be struck, one that recognises the real commercial needs and problems of the company as well as our members' legitimate interests," he said.
"But we are not and never will be prepared to see our members and our union humiliated, victimised and reduced to ruins."
He further claimed that BA's bosses were not interested in reaching an agreement.
"They are trying to starve our members back to work," said Mr Woodley.
BA denies the bullying accusations, and on Sunday said it had reintroduced all flights from Heathrow to New York's JFK airport and that it would add more.
It insisted its operations went "very well" during last week's action and that it had made another "good start" to the second bout of walkouts that began on Sunday.
"We have announced a larger schedule at Heathrow for this round of strikes, because of the numbers of crew reporting for work," it added.
BA's services from London's Gatwick and City airports continue to be unaffected.
The Unite union claimed BA cancelled more than 100 flights on Sunday and said its strikes continued to be strongly supported.