LONDON — A three-day strike by British Airways cabin crew affecting thousands of travellers is set to start at midnight tonight after last-ditch talks between the airline’s management and union leaders collapsed.

BA Chief Executive Willie Walsh said it was “deeply regrettable” that the union declined to accept a proposal on pay and working conditions from the airline, adding that offer would be withdrawn once the strike begins.

BA, which placed an advertisement in British newspapers reassuring customers it would do its best to minimize disruption, said it expects to operate around 65 percent of its scheduled flights over the next three days.

A total of 1,100 flights out of the 1,950 flights scheduled to operate during the walkout will be canceled, but the airline has leased planes and crew from rival carriers to take up some of the shortfall.

At its Heathrow base, more than 60 percent of long-haul flights will operate, but only 30 percent of short-haul. At Gatwick, all long-haul flights and more than half short-haul flights will run as normal.

“Tens of thousands of BA people stand ready to serve our customers,” Walsh said. “BA will be flying tomorrow.”

Picket lines will be mounted over the weekend at several entrances to Heathrow, but Walsh said he had “no concern whatsoever” about the threat of solidarity actions in other countries. Unions representing ground crew and plane service workers in the United States, Spain and Germany have indicated they may be prepared to join a walkout.

Unite joint general secretary Tony Woodley said that BA “does not want to negotiate and ultimately wants to go to war with this union.”

The prospect of travel chaos in Britain in the run-up to the Easter break also intensified with news that railway signal workers voted in favor of a strike, joining rail maintenance workers. The Rail Maritime and Transport union has not called dates for a walkout of both groups of workers but has refused to rule out the Easter long weekend.

Easter is April 4.

The prospective industrial rest is an unwelcome turn of events for the ruling Labour Party on the eve of a national election.

“As the country struggles out of the recession, the last thing we need is the unions holding the country to ransom,” said Theresa Villiers, the opposition Conservative Party’s transport spokeswoman.

BA argues that the disputed changes — including a pay freeze in 2010, a switch to part-time work for 3,000 staff and a reduction in cabin crew sizes from 15 to 14 on long-haul flights from Heathrow airport — are critical for its survival. Unite argues it was not properly consulted on the changes.

BA said today it had offered to modify the changes, even though they had been approved by Britain’s High Court, but Unite had declined to put the offer to its members to vote.

Analysts estimate that BA has already lost more than 25 million pounds because of canceled tickets and the cost of contingency plans, which include leasing fully crewed planes from other airlines.

Unite has a second, four-day walkout planned to begin on March 27 and has said more strikes will be scheduled for after April 14 if the dispute is not resolved. It has made a pledge not to walk out over the busy Easter period.