Heathrow and other airports report no border delays
Heathrow Airport has reported no significant delays in processing arriving passengers during the public sector strike, despite warning earlier this week of possible 12-hour hold-ups.
Passengers at Stansted Airport have also reported no delays.
Home Office minister Lord Henley said border security was the "top priority" and contingency plans were in place.
Ministers say no border controls will be relaxed to ease queues at airports during the strike.
On Tuesday afternoon, Lord Henley, answering an urgent question in the House of Lords, said: "We are satisfied that security will be maintained.
"We started training additional staff in contingency in April and adequate resources are now available. Any staff deployed on the front line will have received training required to operate effectively. Arriving passengers will remain subject to checks at the border by appropriately trained staff."
The UK Border Agency, which is part of the Home Office, had examined a range of options including whether service personnel could be used at Heathrow. However, officials concluded they could manage the airport with volunteers from within the civil service.
Hayes and Harlington MP John McDonnell, whose constituency includes Heathrow, had called for the airport to be shut on Wednesday on safety grounds after its operator BAA warned of possible "gridlock" and 12-hour immigration delays.
Labour's Mr McDonnell said it was clear large numbers of staff at the London airport would be joining the strike.
Although attempts had been made to bring in extra people, it still meant that safety and security would be "in the hands of workers who are untrained in airport security or, at most, have had an inadequate couple of days' training," Mr McDonnell said.
Lucy Morton, deputy general secretary of the Immigration Services Union, said she regretted its members were striking.
She said: "Nobody wants the public caught up in this, least of all this union. This is not something that we have done lightly."
She said the union members had been left "feeling there is nowhere else to go" and this was the only way "to make government listen to the people on the front line".