Willie Walsh, the BA chief executive, said: "It's deeply regrettable that a proposal that we have tabled to Unite that I believe is fair... has not been accepted."
However, he admitted that it was lower than previous offers as a result of the costs the airline has incurred due to the planned industrial action.
A total of 1,100 flights out of the 1,950 flights scheduled to operate during the walkout will be cancelled, but the airline has leased aircraft and crew from rival carriers to take up some of the shortfall.
At its Heathrow base, more than 60 per cent of long-haul flights will operate, but only 30 per cent of short-haul.
"BA will be flying tomorrow and will continue to fly throughout these periods of industrial action"
BA chief executive
At Gatwick, all long-haul flights and more than half the short-haul flights will run as normal.
"BA will be flying tomorrow and will continue to fly throughout these periods of industrial action," Walsh said.
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.
The airline wants to save more than $90 million annually through a series of cuts to pay and staffing to help cope with a fall in demand, volatile fuel prices and increased competition from low-cost carriers.
Unite argues it was not properly consulted on the changes.
BA says 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.
More strikes planned
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.
The industrial action is an unwelcome turn of events for Gordon Brown, the prime minister, with parliamentary elections expected within months, as his ruling Labour party is supported by Unite.
Analysts estimate that BA has already lost more than £25 million because of cancelled tickets and the cost of contingency plans, which include leasing fully crewed aircraft from other airlines.
"The prime minister believes that this strike is in no one's interest and will cause unacceptable inconvenience to passengers,'' Brown's office said in a statement.