Strikes have been set for three days from March 20 and for four days from March 27.

Unite initially offered hope that the walkouts could be averted if its members voted in favour of an eleventh-hour proposal tabled by the airline.

But BA appeared to kill off those hopes, saying the union rendered that proposal invalid when it went ahead and announced industrial action.

BA chief executive Willie Walsh said: "Evidence of the gap between the two sides is the cynical action of Unite in announcing strike dates.

"We have done everything we can to reach an agreement with this trade union, but it has proven impossible to get them to grasp the full scale of the problems we face."

Len McCluskey of Unite said the airline's withdrawal of the offer "beggared belief".

He added: "This move seems to put it beyond doubt that management is bent on confrontation and never had any intention of an agreement.

"BA are now stopping their own employees from having a voice on an offer that could have ended this dispute."

It is the latest twist in a long-running and bitter dispute over cost-cutting plans that have seen the airline impose changes to cabin crew working practices.

Unite announced the strike action on Friday morning after discussions aimed at achieving a negotiated settlement finally broke down.

Following the strike announcement, BA assured passengers due to travel between 19 and 31 March that they could either re-book or get a full refund.

The airline said it was "extremely disappointed" with the union's plans, which it said had "no shred of justification".

BA has drawn up contingency plans to deal with the strikes, with up to 1,000 volunteer staff ready to work as cabin crew, including hundreds of pilots.

The airline has also said it will hire 23 fully-crewed planes from charter companies to help run flights from Heathrow.

Flights from London's City Airport, including long-haul services to New York, would operate normally in the event of a strike.

Meanwhile 70% of cabin crew would work at Gatwick, meaning all long-haul and 50% of short-haul flights would be unaffected, BA has said.

No specific details were given about flights from Heathrow in the event of industrial action, but Willie Walsh said a "substantial" number of long and short-haul services would still operate.

The blow comes after a new front opened up in BA's industrial relations battle, with Sky sources warning of a potential dispute with ground staff over proposed changes to working practices.

Talks with some 4,500 ground staff had been taking place for around a year but took a new twist when the airline wrote directly to staff earlier this month with details of new contracts.

"Unite sees the letter as a provocative attempt to bypass negotiations and impose change rather than negotiate," Sky News business correspondent Joel Hills said.

The union has told BA that if the airline does not withdraw the letter, it will launch a "consultative ballot" for ground staff.

This would not amount to a strike ballot, but members would be asked whether they would like to vote on industrial action.

A BA spokesman said: "We are consulting with our ground handling staff at Heathrow about potential changes to improve the way in which we work.

"Any talk of a ballot for industrial action is speculative and premature."