A “nationwide issue” with Border Force e-Gates has caused significant disruption at airports.
Employees at Heathrow, Gatwick and Manchester are all helping out while officials try to fix the problem.
It means long queues for passengers arriving tonight, including at Edinburgh and Stansted.
Last week, Border Force workers staged a four-day strike at Heathrow in a dispute over working conditions.
A Heathrow spokesperson said: “Border Force is currently experiencing a nationwide issue which is impacting passengers being processed through the Border. Our teams are supporting Border Force with their contingency plans to help resolve the problem as quickly as possible and are on hand to provide passenger welfare. We apologise for any impact this is having to passenger journeys.”