Hundreds of thousands of air passengers faced travel chaos today with many flights cancelled and Heathrow closed to incoming short-haul services.
British Airways cancelled all short-haul flights in and out of Heathrow. Passengers due to travel on easyJet and Ryanair flights from Stansted were told to leave the airport and rebook flights.
A Luton airport spokesman said there were delays on most flights and that all remaining easyJet flights departing from Luton had been cancelled for the rest of today.
Many foreign airlines suspended services to London even before Heathrow airport operator BAA announced a suspension of short-haul Heathrow-bound flights that had not already left for London.
Those axing flights to London included Italian carrier Alitalia, German airline Lufthansa, Spanish carrier Iberia and Irish airline Aer Lingus. Air France scrapped five of its Heathrow to Paris flights.
British Airways said it had cancelled all its short-haul flights to and from Heathrow for the rest of today. BA added that it was also cancelling some domestic and short haul services in and out of Gatwick Airport during the remainder of the day.
The airline said it hoped to operate as many long-haul services as possible from Heathrow and Gatwick, but these flights would be subject to delay.
Short haul and domestic services operated by the airline's regional subsidiary BA Connect to and from UK regional airports (not Heathrow and Gatwick) were continuing to operate, albeit with delays.
Around 400,000 people in the UK were affected by the travel chaos today, airline information company OAG estimated.
It said that more than 3,800 flights should have taken off in the UK today and that around 3% of today's planned flights from UK were transatlantic. In total, more than six million people are expected to travel from Heathrow this month.
OAG said that worldwide, airlines had planned to operate 81,958 scheduled flights today, offering around 9.5 million seats.
More than 650 flights were scheduled to depart today from Heathrow, with 76 of these destined for a US airport. Of the 59 airports in the UK, nine operate transatlantic flights.
Stephen Nelson, chief executive of airport operator BAA, said it was the first time that airports had "faced a security mandate of this scale and severity".
He added that there were "very severe delays" at all BAA's seven UK airports, but all were "open for business".
He said the ban on short-haul incoming flights at Heathrow was in place until 3pm and would then be reviewed.
Transport Secretary Douglas Alexander said the seriousness of the terrorist threat had given authorities "no choice" but to implement today's security measures.
He said he recognised the measures were already affecting many families and individuals who were travelling by air and that all efforts were being made "to address these operational challenges".
Urging air passengers to "show understanding", Mr Alexander said consideration was being given to the long-term response that would be necessary.Reuse content