Hundreds of thousands of passengers booked to fly to, from or through Heathrow airport on Monday and Tuesday still do not know if they will be grounded.
More than 2,000 workers including security guards, firefighters and engineers are planning to stop work in a pay dispute.
The main concern of airlines and the airport is that queues for security may become unmanageably long.
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While talks are continuing, the airport says 177 flights have been cancelled. But airlines have not yet said which will be hit.
Some travellers have expressed their frustration on social media.
“JackFlash” tweeted Heathrow to say: “If my travel plans are disrupted as a result of your lack of information regarding next week’s strike action, I will hold you liable for any and all costs as well as any inconvenience.
Heathrow airport began its response with what, in an aviation context, is an unfortunate phrase.
“Hi Jack,” it started. “We sincerely apologise for this. However please do remain in contact with your airline as they will have the latest information for you regarding the status of your flight.”
The complainant responded: “No, my airline are not responsible for the employees undertaking strike action, you are.
“Therefore you are legally responsible for updating users of the airport – ie passengers. Which flights are affected?”
Heathrow Airport replied: “Hi Jack, unfortunately we do not have any information on this. Your airline will know which flights and statuses that have changed for their passengers and will be able to advise you further. Thank you.”
The Independent has been seeking more information from key airlines about their plans.
Virgin Atlantic is moving some flights from Heathrow to Gatwick. Some inbound passengers on Sunday evening departures from the US will touch down at the Sussex airport, so that Monday’s flights can take off on schedule – albeit from Gatwick.
Flybe is planning the highly unusual step of flying aircraft from Heathrow empty, to avoid the risk of delays.
Four services to Edinburgh and one to Newquay may be cancelled over the two days; the aircraft will fly out normally, but return with a full complement of passengers.
British Airways operate more than half the flights in and out of Heathrow. A BA spokesperson said: “We have not currently confirmed any cancellations and we will be updating customers in the course of Sunday.
“Customers booked to travel on short-haul services departing from Heathrow on either proposed strike date can re-book to BA flights from Heathrow before the proposed industrial action [4 August] and from 7 to 12 August.
“They can also rebook on BA flights from Gatwick or London City.”
The airline says it will “update our customers with as much notice as possible” on Sunday.
The writer Marcus Williamson, currently on holiday in the Peloponnese, is booked on British Airways from Athens to Heathrow on Tuesday 6 August.
He has written to Heathrow airport saying: ”You’re spoiling my holiday in Greece because I’m having to worry about getting home on time whilst your directors get paid millions and your staff are not paid nearly enough for the important work they do.
“Please sort out the pay issue today and treat everyone fairly. There’s clearly plenty of money around to pay fair wages to everyone.”
Heathrow airport is telling passengers: “Heathrow remains committed to finding a solution with Unite to avoid further strike action.
“This is a difficult time for Heathrow, our airline partners, and above all our passengers, as the proposed strike action has been unreasonably designed to impact passengers taking well-deserved holidays.
“We apologise for any disruption to your journey, and appreciate your patience during this time.”