British Airways cabin crew are to stage a series of strikes in a bitter row over cost-cutting, threatening travel chaos for tens of thousands of passengers, it was announced today.

Unite said its members at the airline will walk out for three days from March 20 and for four days from March 27 following the collapse of talks aimed at resolving a long-running dispute over jobs, pay and working conditions.

But Unite also announced it will ballot its 12,000 cabin crew members on a new offer tabled at the 11th hour by the airline, giving a glimmer of hope that the strikes can be averted.

Cabin crew will vote electronically on the new offer and the result is expected next week, before the first strike is due to go ahead.

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, while 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 chief executive Willie Walsh said a "substantial" number of long and short-haul services would still operate.

Mr Walsh has made clear that changes, including reductions in cabin crew, would not be reversed.

A planned 12-day stoppage by 12,500 cabin crew over Christmas was halted after a successful legal challenge from BA, and the union ruled out striking over Easter.

The two sides held talks under the chairmanship of TUC general secretary Brendan Barber, which broke up without agreement on Wednesday evening, although channels of communication have remained open.

The cabin crew branch of Unite - Bassa - said yesterday it had no wish to cause disruption to BA and its customers and urged Mr Walsh to reconsider an offer tabled during this week's talks.

Bassa said the difference between the two sides' cost-saving proposals was £10 million, adding that far more than that has already been spent by BA on contingency plans for a strike.

Unite put forward a 10-page document which detailed a series of savings on pay, crew numbers, natural wastage, hotel costs and meal allowances, totalling almost £63 million.

A union source said he believed the offer was "fair, far-reaching and generous" and met BA's financial demands while giving safeguards on pay and conditions to existing crew.

It is believed that under Unite's proposal, BA would reinstate a 15th crew member on long-haul flights, something the airline has been strongly resisting.

Unite was understood to be offering a one-year pay freeze, followed by a 2.6% pay cut, then an increase in line with RPI inflation or 2.6%, whichever is higher.

A lump sum bonus worth around £7 million would be paid at the end of year three.

BA said its package would save £62.5 million a year and would not reduce the pay of existing crew.

The airline said the union's proposals fell "significantly short" of this level of savings and would lead to pay cuts of between £1,000 and £2,700 for crew, figures the union disputed.

Shadow business secretary Ken Clarke accused ministers of failing to condemn the strike because Unite was a major donor to Labour Party funds.

"There is no point in being naive. The fact is that Unite has given £11 million to Labour funds over the past four years.

"They own the Labour Party, which is why there is no condemnation of any kind coming for a particularly irresponsible strike.

"They are totally silent because their silence has been bought."

Len McCluskey, Unite's assistant general secretary, said BA had turned down a "remarkable" offer from the union which he maintained would have given the airline everything it wanted.

"This offer gives the lie to the smears that these skilled and professional employees are mindless militants indifferent to BA's difficulties or are defending a privileged position.

"The company contests our calculations but quibbles over the precise costing of our concessions pale into insignificance compared to the losses BA will sustain in the event of an industrial dispute."

Mr McCluskey said it was right that cabin crew should vote on the new offer although he said it fell short of what the union believed was needed to resolve the dispute.

Unite will make no recommendation on the offer and warned that further industrial action will be held after April 14 if the dispute remains unresolved.

Rochelle Turner, head of research for Which? Holiday magazine, said: "Once again BA passengers are left in limbo. Those due to travel with the airline over this period are unlikely to get a refund on their ticket until flights are officially cancelled, and risk losing money if they book a second ticket through another airline.

"If you are due to fly with BA during the strike, go to the airline's website for the latest information before you leave home to avoid a wasted trip.

"Passengers whose flights are cancelled should get a full refund, or may be able to rearrange their BA flight date, but that won't compensate for the time and money lost on hotel bookings or tour dates."

BA said it was "extremely disappointed" by the decision to strike and added that the action would cause "massive disruption".

"Unite's action has no shred of justification," said a spokesman.

"British Airways' crew are rightly renowned for their professionalism and skills. Our entire package for crew recognises that and is reasonable and fair.

"British Airways is facing two years of record financial losses. Unlike other businesses, we have avoided compulsory redundancies and made changes designed to secure a long-term future for our company and our staff. Cabin crew face no pay cut or reduction in terms and conditions - and remain the best rewarded in the UK airline industry."

The company said it would do "everything we can" to protect the travel plans of its customers.

"We plan to operate all British Airways' flights from London City Airport, including long-haul services to New York.

"From Gatwick, we plan to operate all long-haul services and about 50% of short-haul.

"From Heathrow, we plan to operate a substantial part of our long-haul and short-haul schedule," said the spokesman.

"We are also in the process of obtaining seats on flights operated by other carriers to enable thousands of customers to fly to their chosen destinations."

The spokesman said a revised flying programme would be announced about five days before the strike is due to begin.

But he stressed BA would be "available" to hold talks with Unite at "any time".