British Airways cabin crew have started the first of four five-day walkouts.
The industrial action, which began at midnight, will primarily affect long-haul services in and out of Heathrow. Some short-haul services will also be affected.
The airline's London City and Gatwick services will run as normal.
Unite union boss Tony Woodley said yesterday that the strike would be called off if Willie Walsh, BA's chief executive, reinstated travel concession withdrawn from staff who went on strike in March.
Walsh said employees' travel perks would be given back, but seniority and length of service would no longer give them priority boarding on flights. In effect they would be treated like new employees.
The union called Walsh "vindictive" and said the offer fell well short of what members were demanding.
Talks between BA and the union were abandoned on Saturday afternoon after protestors from the Socialist Workers Party stormed the meeting room.
In spite of the chaos, both Walsh and Woodley said some progress had been made and that they expected talks to continue this week.
The dispute between cabin crew and BA started more than a year-and-a-half ago when the airline announced cuts to pay and conditions for staff in a bid to lower its cost base.
This latest strike, according to the union, centres round the withdrawal of travel perks.
Walsh has publicly blamed Bassa - the cabin crew branch of Unite - for the failed negotiations.
Two more five-day strikes are scheduled to being on May 30 and June 5.