British Airways cabin crew are launching a fresh wave of four strikes in a long-running dispute over pay.
Unite said its members at the airline were "piling on the pressure" by walking out from today, and again for four days from February 22.
Members of the so-called mixed fleet have taken 11 days of action so far this year in protest at "poverty" pay.
A BA spokesman said: "As during the previous rounds of strikes, we will fly all our customers to their destinations.
"All flights to and from London Gatwick and London City will operate as normal, as will the vast majority of flights to and from London Heathrow.
"To enable all customers to travel, we will be merging a very small number of flights at Heathrow - about 1% of total flights planned.
"Our pay offer for mixed fleet crew is consistent with deals accepted by more than 90% of British Airways colleagues, most of them represented by Unite.
"It also reflects pay awards given by other companies in the UK and will ensure that rewards for mixed fleet remain in line with those for cabin crew at our airline competitors.
Unite regional officer Matt Smith said: "For every hour British Airways 'wet leases' an aircraft from another airline to cover striking cabin crew it costs in the region of £2,000 to £3,000.
"Our estimates put the amount of money British Airways has spent on defending the dispute and poverty pay at £1 million.
"This is money which the airline has taken a conscious decision to give to other airlines rather than addressing pay levels which are forcing hard-working mixed fleet cabin crew into financial hardship.
"We would urge British Airways to reconsider its costly intransigence and enter talks at Acas and reach an agreement."
The two sides have clashed over pay, with Unite saying the cabin crew earned an average of £16,000 a year, including allowances, but BA insisting no one was paid below £21,000.
The mixed fleet, who work on short and long-haul flights, joined BA since 2010.