British Airways has been urged to "sort out this mess" after an email error meant some passengers were wrongly told their flights were cancelled.
Consumer group Which? said the airline should take immediate action to rectify the mistake, which followed the announcement that pilots will strike for three days next month.
British Airways apologised after it emerged some passengers - the exact number has not yet been confirmed - were wrongly told their flight would be cancelled and they were advised to rebook.
Adam French, consumer rights expert at Which?, said the issue had caused "a lot of confusion and anxiety".
He said: "It is vital that the airline ensures that any customer who was initially informed that their flight was cancelled and has booked an alternative flight is not left out of pocket.
"British Airways must now put all resources necessary into sorting out this mess as soon as possible."
The airline said on Saturday that it would be getting in touch with affected customers to clarify the situation.
"We are sorry that some customers received an email in error to say that their flight had been cancelled on non-strike days," a spokesman said.
"We are sorry for any confusion and inconvenience this has caused."
He said anyone who went on to spend money on alternative arrangements should contact British Airways "and we will deal with each case on an individual basis".
The spokesman clarified that some flights on non-strike days will be affected, but that the bulk of those cancelled are on the strike days of September 9, 10 and 27.
The British Airline Pilots Association (Balpa) announced the strikes on Friday after 93 per cent of its members rejected an 11.5 per cent pay rise across three years.
Swathes of passengers on flights which will be grounded tried in vain to contact the airline to make alternative arrangements on Saturday.
One said his 10-year wedding anniversary "trip of a lifetime" was in doubt after more than 100 calls to BA to get a refund went unanswered.
Scott Kishere, a filmmaker and police community officer, said: "My wife and I have been told that our flight to Toronto has been cancelled.
"This was a full package we booked through BA for our 10-year wedding anniversary. I have made over 100 attempts to contact them for a refund and they're not answering.
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"Until we get a refund we do not have the money to rebook our trip of a lifetime.
"With the days counting down it now looks unlikely we will be able to celebrate our 10-year anniversary as expected."
Balpa said more strike dates could yet be announced, adding that they were "a last resort and with enormous frustration at the way the business is now being run".