British Airways cabin crew mounted picket lines outside airports for the second weekend in a row today as another wave of industrial action began in a bitter row over jobs and cost cutting.
Unite said early indications were that its 12,000 members involved in the dispute were strongly supporting the industrial action.
Hundreds of strikers gathered at a football ground close to Heathrow before being taken to several picket lines around the airport.
The airline said it will fly more than 75 per cent of customers booked to travel during the four days of strike action and expects to handle more than 180,000 of the 240,000 people who had planned to travel from March 27-30.
A number of short haul flights left Heathrow this morning and there were arrivals from overseas destinations.
Most long haul services don't leave Heathrow until later in the day.
BA said around 18 per cent of its passengers were re-booked to travel on other carriers, or changed the dates of their BA flights to avoid the strike period.
The two sides continued to clash on the cost of the dispute, with Unite claiming BA will be left with a bill of £100 million because of the current strikes and last week's three day walkout, twice the £7 million a day that BA told the City about earlier this week.
BA reiterated that its "current best estimate" of the cost of the first round of strikes was £7 million a day, and that assessment of the cost of subsequent strikes would only be possible after they had taken place.
Unite officials said today that BA had leased fully crewed planes from eight companies, and was again using volunteer pilots and management to stand in for striking cabin crew.
The airline said several thousand customers brought forward their departures to avoid the impact of the strike. BA said that, over the next four days, it would fly a full, normal schedule from Gatwick and London City Airports.
At Heathrow, BA said it would operate 70 per cent of its long-haul programme (up from 60 per cent in the first strike period from March 20-22) and 55 per cent of its short-haul programme (up from 30 per cent).
BA chief executive Willie Walsh said: "The vast majority of BA staff, including thousands of cabin crew, are pulling together to serve our customers and keep our flag flying.
"At the same time, I feel really sorry for those customers whose plans have been ruined by the Unite union's completely unjustified action. Despite the union's promises, this strike has affected the Easter holiday plans of thousands of hard-working people."
Mr Walsh stood firm on the airline's decision to withdraw travel perks from striking cabin crew, saying that staff knew they would lose their travel concessions if they joined the three-day walkout last weekend. Unite accused BA of "unacceptable anti-union bullying" by taking away the travel perks, but Mr Walsh denied this.
The union has insisted that any peace deal must now include giving back travel concessions to cabin crew, as well as reinstating a number of staff who have been suspended as a result of the dispute.
Mr Walsh rejected suggestions the withdrawal of concessions was a "punishment" or attempt to "break the union", adding: "We told them about the consequences if they went on strike."
BA said there will be cancellations at Heathrow throughout the four days and for two days after the strike has ended due to aircraft, pilots and cabin crew being out of position.
"British Airways will do all it can to minimise the numbers of cancellations and will reinstate flights wherever possible if cabin crew come to work as normal.
"Customers were informed of cancellations five days ago and almost all of them will be able to fly on British Airways over the four days, change the date for another BA flight or travel with another airline where available. In some cases customers have also claimed a full refund.
"On Saturday morning our operations at Gatwick, where we aim to fly all flights as planned, have got off to a very strong start. At Heathrow we have got off to a good start. London City continues to operate as normal.
"Cabin crew are reporting as normal at Gatwick, just as they did last weekend during the first strike period. The numbers of cabin crew reporting at Heathrow are currently at the levels we need to operate our published schedule.
"This is the second part of the biggest contingency plan we have ever launched and our aim will continue to be to fly as many customers as we can," the airline said.
Unite said it believed BA was grounding its own flights so it could use pilots as cabin crew on other BA flights.
Len McCluskey, Unite assistant general secretary, said: "This is the great BA con trick. It's a three-card trick Paul Daniels would be proud of.
"BA is claiming that it can function but it is doing so by throwing away millions of pounds every day as it dumps its passengers on other carriers. Passengers who turn up expecting to fly BA, a brand they trust and have paid a premium for, will now be shipped on to carriers they've never heard of.
"And instead of fully trained professional crew, they'll be attended to by a ragbag bunch of pilots, managers and strike-breakers masquerading as crew.
"In its desperation to break its workforce, BA is inflicting another trashing on this brand."