Air passengers travelling to the US were told to expect delays after heightened security measures were put in place in the wake of the failed bomb attack on a flight to Detroit.
People flying across the Atlantic were advised to arrive early and carry just one piece of hand luggage.
After checking in and going through security as normal, passengers can expect extra bag checks and body searches at the departure gate before boarding flights. And travellers were being told to remain seated for the final hour of the journey, with no access to the bathroom or overhead lockers, and nothing on their laps.
Airport operator BAA said: "Passengers travelling to the United States should expect their airline to carry out additional security checks prior to boarding. To support this important process, which will take time, we would advise passengers to leave more time to check in and limit the amount of baggage being taken on board the aircraft.
"If in any doubt, please contact the relevant airline for further information."
General airport security remained the same for passengers travelling outside the US, with no change to luggage and liquid restrictions. Both British Airways and Virgin Atlantic posted statements on their websites about the revised security guidance.
BA said: "Passengers travelling to the US from Heathrow, Gatwick or London City are advised that due to additional security screening they are only allowed to take one item of hand luggage on board. Customers are advised to check the status of their flight before they leave for the airport. Customers travelling to other destinations outside the United States are not affected."
The airline said any US-bound passengers with wrapped presents in their hand luggage would be made to unwrap them at the gate.
Virgin Atlantic warned of possible delays and said: "In order to minimise delays and congestion, we are asking all passengers to arrive at the gate early. Hand baggage allowances for all passengers flying into all US airports has been reduced to only one item of hand baggage."
Transport secretary Lord Adonis said: "We have agreed enhanced search regimes for passengers on flights to and from the United States. These have taken place immediately. They will lead to delays in flights to the US."
