The unions have chosen the strike date shrewdly.

The walk-out of UK Border Agency staff, in protest against pension changes, should have no effect on family holidays. Midweek at the back end of November is as low-season as the holiday business gets.

But the economic damage will be substantial, particularly to the lucrative transit traffic through Heathrow. The action injects an extra dose of uncertainty into what is already an uncertain business. BA is warning of a "significant risk of severe delays in passing through passport control, with potential knock-on effects on wider airport operations".

This is the Heathrow scenario the airlines fear at dawn on Wednesday: as the first wave of flights from North America, Asia and Africa arrives, the queues for immigration quickly build. Soon, airlines are instructed to keep passengers on newly arrived flights on board. The stands are thus blocked to later inbound flights, which are obliged to divert to foreign airports rather than adding to the rapidly building queues.

Departing passengers, meanwhile, have nowhere to go, because their planes are still occupied. With around 10,000 passengers turning up every hour in the hope of flying, the "landside" areas soon degenerate into the kind of squalor last seen at the 1970 Isle of Wight pop festival. Passengers should hope for the best but prepare for the worst.