International travelers returning home after Christmas or jetting off for New Year breaks in the US are being warned to expect delays, increased security measures and baggage restrictions in the wake of an attempted airliner bombing on December 25.

The US Transportation Security Administration has ordered additional security screening for all inbound flights to the US, including pat-downs by security staff and increased bag searches. Restrictions vary between airlines and airports, and passengers are advised to check with their airline ahead of travelling and to arrive early at the airport to allow extra time for the enhanced security screening. Several airlines, including British Airways, Air Canada and Virgin Atlantic, have announced hand baggage restrictions of one item, permitting passengers to check extra hold baggage free of charge.

Airlines have also warned that any gift-wrapped presents carried as hand luggage by passengers will need to be unwrapped at the airport on US-bound flights. Some airlines have also advised passengers to bring extra entertainment for children, as cabin entertainment systems may be disabled for parts of the flight. Hong Kong carrier Cathay said it was banning US-bound passengers from using cabin phones at any time during the flight.

Around 4 million international passengers will have entered the US every month during 2009, according to the Office of Travel and Tourism Industries, the majority by air. London Heathrow, the world's busiest airport, has over 100 daily scheduled flights  to the US alone, carrying an estimated 25,000 people.