Tui has made an apology following a series of lengthy flight delays this week.
The operator insisted it was “doing everything” to limit delays to flights.
Some holidaymakers had reportedly been forced to wait for more than 40 hours for a flight from Cardiff airport to Zante.
Other flights to Ibiza and Dalaman are reported to have faced delays of more than 24 hours.
And the Unite union warned that delays at most major airports during the Easter holidays will be repeated next week during the half-term holidays and will be “even worse” during the summer.
Unite general secretary Sharon Graham said: “During the pandemic, when airline operators and others in aviation slashed jobs to boost corporate profits, we warned this corporate greed would cause chaos in the industry. The aftermath of mass sackings is now chronic staff shortages across the board. Aviation chiefs need to come clean with the public. This is a crisis of their making.
“We are determined that workers will not pay for this crisis. Current pay and conditions in the industry are so poor that workers are voting with their feet. It can only be resolved by offering higher wages and better working conditions for staff. Unite is utterly determined to fight for that.”
Tui said in a statement: “We would like to apologise to some of our customers who have experienced flight delays in recent days.
“As the UK’s biggest holiday company, our priority is to take customers on holiday safely, so we will never cancel holidays.
“We continue to work closely with our airport partners to monitor the situation and continue to provide the best possible holiday experience for our customers.”
The company added: “We are doing everything we can to limit any flight delays, which can happen from time to time for various operational reasons.
“On the rare occasion flight delays do take place, we will do everything we can to support our customers, through direct communications and providing overnight accommodation and transfers where needed, as well as meals and refreshments.”
Tui reminded travellers not to arrive at the airport too far in advance of their flight, “as this can contribute to terminal queueing delays”.
The operator said: “Standard check-in opens at least two and a half hours before departure time for short and mid-haul flights, and at least three and a half hours before departure for long-haul flights.
“At some airports, we are able to offer day before bag drop and self-service bag drop.”