All British Airways flights from Heathrow have been suspended after staff walked out in support of sacked catering workers.
The sacked staff were not employed by BA, but by flight meals supply firm Gate Gourmet.
BA baggage handlers, loaders and bus drivers have walked out in sympathy with the catering workers.
Gate Gourmet sacked 800 staff yesterday in a row over plans to restructure the company. They were sacked after what employers called an unofficial strike.
BA has been providing passengers with food bags and vouchers and may compensate some customers.
The Transport and General Workers' Union said it was trying to begin talks with Gate Gourmet to resolve the issue as soon as possible.