The first day of a strike by passport office workers hit holidaymakers.
Union officials claimed more than 2,000 passport staff left their posts in a row over a 2.5% pay offer and office closures.
The three day walk-out affected the UK's seven regional passport offices and 68 interview offices, the Public and Commercial Services Union (PCS) said.
But the Identity and Passport Service (IPS) put the number of absent staff at 1,046, and said all premises remained open apart from one interview office.
The stoppage - which came at the start of the school holidays - means that anyone who discovers their passport has expired may have to miss their flight.
For an extra charge, new documents can usually be obtained in one day but this service cannot be guaranteed during the strike. Travellers waiting for ordinary passport applications to be processed will also face delays.
The PCS said the walk-out had seen "strong support" with eight out of 10 members staying away.
All appointments were cancelled at one regional office, Liverpool, and only a "small number" of staff turned up for work across the other six, it claimed.
PCS general secretary Mark Serwotka said: "Strike action is not a step which is taken lightly and the inevitable disruption caused over the three days is regrettable. However, those on strike have been pushed to the limit by a management seemingly hell bent on unpicking a world class service and a government content on paying poverty wages with its policy of below inflation pay.
"Passport staff deliver an internationally renowned service, yet have to put up with pay cuts in real terms, draconian working practices and office closures which will damage the service the public receive."
