British Airways has lost up to £100 million in market value since the lightning strike at Heathrow began.
Baggage handlers and other Heathrow staff came out in sympathy of 300 workers at a private catering firm who were sacked.
The workers had refused to sign new contracts that the TGWU said would have forced them to accept a change in working practices and pay that was not in their interests.
The airline has cancelled all flights until at least 6pm on Friday, but sources expect the chaos to continue beyond that time.
CEO Rod Eddington - who is leaving in September - has called on union and management of Gate Gourmet to meet and try to resolve their differences.
That is seen as a problem because the TGWU says it will not hold talks until workers are given their jobs back, and Gate Gourmet says that is not going to happen.
The union accuses the US-owned firm of trying to bring in American "union-busting" antics.