The latest five-day walkout has been started by cabin crew of British Airways at midnight as their dispute centred on pay, jobs and conditions continues.
The walkout will coincide with half-term holidays, and could be followed by more action starting on 5 June.
British Airways announced that it hopes to operate more flights in the coming days than during the previous disruption last week.
The Unite union says it would suspend strike action if British Airways restored travel perks it took from strikers in March.
It claims BA has lost £84m through the industrial action so far.
Negotiations ended without agreement on Friday, with Unite blaming BA’s chief executive Willie Walsh for the deadlock. Conciliation service Acas will be contacting both sides to arrange further talks.
Unite’s joint leader Derek Simpson said of Mr Walsh: “He has refused to reinstate travel concessions in full, despite Unite making it clear that the union would suspend the strike if he did so.”
“It would cost BA nothing to reinstate these travel concessions – yet Willie Walsh is prepared to see the strike continue, putting the travel plans of thousands of passengers at risk and costing BA over £100m over the course of this dispute.”
BA staff returned to work for one day between the two strike periods, but the airline said that although the “vast majority” of flights were operating it was impossible to run a full service.
A BA spokeswoman said: “All flights will operate at Gatwick and London City over the weekend.
“The knock-on impact at Heathrow is far less than anticipated, due to the numbers of cabin crew who came to work as normal over the past week.”
This week at Heathrow, BA said the amount of long-haul flights would be increased from 60% to 70% and short-haul flights from 50% to 55%.
Unite said 121 flights out of 333 scheduled to leave Heathrow on Friday were cancelled.
But in national newspaper adverts published on Saturday, BA said it had flown 325,000 passengers during the week. It said it flew to 87% of its long-haul and 100% of its short-haul destinations during the walkout.
The dispute is set to dominate Unite’s annual conference in Manchester, where about 700 delegates will gather on Monday.