Blame the government. Blame airline schedules. Blame the wind.
Witnesses at a British parliamentary committee hearing on Tuesday offered a variety of reasons for the long waits some arriving passengers endure at immigration checks at London's Heathrow airport.
Recent reports of people standing in line for two hours or more at Heathrow airport, the nation's busiest airport, have raised concerns not only about management but about the country's image.
British immigration minister Damian Green said border agents were sometimes overwhelmed when planes arrive with more passengers than expected.
For commercial reasons, airlines also like to arrive early in the morning, particularly on North Atlantic routes, Green said.
Arriving passengers can be victims of unpredictable factors including tailwinds and headwinds which can affect the times when planes arrive, and this in turn determines the order in which passengers join the immigration line, he said.
"That will depend on the wind, over which, with the best will in the world, airlines and the Border Force don't have the control," Green said.
Passengers who are not citizens of European Union countries have been particularly affected by delays in clearance.
Representatives of the airport's owner, British Airways and Virgin Atlantic suggested queues could be speeded up if border agents were instructed to concentrate on identifiable risks instead of subjecting all passengers to the same checks.
"It makes sense to focus resources on the passengers that are the highest risk, for whatever reason _ maybe because of the way they're behaving or the things they're carrying in their bags," said Colin Matthews, chief executive of airport owner BAA Ltd.
But Green said it is "not at all obvious that just having risk-based controls reduces queues."
"They may well involve doing more thorough checks on some of those non-EU passengers."
Union representatives laid most of the blame on cutbacks in staffing, enforced as part of the government's drive to reduce deficits.
Matthews said problems had grown in the last two years.
"I've the impression that the Border Force need to develop their capability in planning," Matthews said.