Airlines stand to lose a lot of money if they carry the wrong kind of passengers. Bringing someone without the right papers to Britain incurs a fine of £2,000. Plenty of foreign countries impose similar penalties. Which is why you find your passport being studied intently when you leave the UK.
"Check-in staff now have to act as junior immigration officers," says an experienced ground manager for a leading airline at Heathrow. "We have to police everything from passport validity to whether or not an onward or return ticket is required. The agent concerned could face a severe reprimand if they let someone board who is later denied entry to the destination country."
Last Sunday morning, Matthew Roberts, a reader from Bristol, arrived at Gatwick in good time for his flight to Faro in Portugal. He had booked a romantic break with his girlfriend to the Algarve coast, staying at a chic boutique hotel.
So meticulous was Mr Roberts's planning that he even thought to check his passport expiry date: 10 January 2012. Knowing that the immigration authorities in some countries stipulate several months of validity beyond the intended date of departure, he checked with the Foreign Office to see if that could pose a problem. Happily, he learned: "Your passport must be valid for the proposed duration of your stay; you do not need any additional period of validity on your passport beyond this."
British Airways, Monarch and TAP Portugal accept the Foreign Office advice, but easyJet does not. The airline's ground staff said Portugal insists on a minimum of three months' validity. Mr Roberts was barred, and the holiday was over – strangled in a tangle of what, given the corporate colour, must be orange tape.
There are 50 ways to miss a flight, and I have tried most of them. But being barred from an EU country with a passport that has 12 weeks to run is a new one on me. Filipe Silva, director of the Portuguese National Tourist Office in London, was equally baffled by easyJet's three-month minimum rule:
"Portugal is part of the European Union. If a British citizen has a passport with only one or two months, they are welcome to visit. There are no limitations."
As 21st-century travellers must wearily accept, these days the passenger is always wrong.
A spokesman for easyJet told me, "We understand Mr Roberts's frustration. As far as we can ascertain, our staff at Gatwick made the correct decision according to the information that we and other airlines rely upon. This case has prompted us to make enquiries with the Portuguese authorities to establish if there has been a change in the rules."
The last relevant change in rules that I can identify took place on 1 January. Not this year, but in 1986 – when Mr Roberts was but one year old, easyJet was still a decade away, and Portugal joined what was then the EC.
As a gesture of goodwill, the airline flew the couple out to Portugal two days later; Mr Roberts did not need to renew his passport. If any airline has treated you the same way, let me know: email@example.com.
The perfect pre-Christmas present: a passport
The best way to avoid that awful airport moment when your holiday plans disintegrate because the airline deems your papers to be out of order: renew your passport nine months before it runs out.
The pace of passport renewals roughly matches the demand for international travel, which slouches along in a trough throughout November and the first half of December. So the men and women who issue these precious documents have the chance to get ahead and reduce the inevitable spike in demand that arrives every summer.
Britain's passport supremo, Sarah Rapson, this week reminded travellers that, if your passport expires before 22 July next year you can renew it now and carry over any unexpired time.
"Passports can be delivered in an average of less than 10 days," says Ms Rapson. Besides eliminating stress, you can also save money: the price of a passport has not changed for over two years, so a rise is due.
Ms Rapson also kindly agreed to consider a suggestion I made: adding an optional box to the passport form for your e-mail address. If you choose to complete it, a reminder could be sent to you nine months before expiry.