Some airlines have prevented passengers from carrying any more than one bag on board, including duty-free purchases.

Passengers have been prevented from using the toilet during the one hour before landing in the US and have been asked to stow all items, including pens, blankets and computers, during the pre-landing period.

In-flight information has also been affected, with some carriers no longer indicating the geographical position of the aircraft while en route in case it could be useful to an attacker.

The ad-hoc measures have affected only flights to the US in what has been a light-touch response so far by US and EU security authorities.

"These measures are designed to be unpredictable, so passengers should not expect to see the same thing everywhere," US Homeland Security chief Janet Napolitano said in a statement on Friday (25 December).

A British-educated Nigerian man tried to detonate a liquid and powder-based bomb on Friday on a flight bound from Amsterdam to Detroit.

The spectre of trans-Atlantic attacks had receded since the last high-profile plot using liquid bombs in 2006.

The period of calm has seen a renewed emphasis on passengers' rights, with European protests over US calls for extra security data or delays to visa waiver programmes for former Communist EU states.

The EU has also avoided the introduction of new body scanners because they invade people's privacy by showing them 'nude.'

The US and UK governments have said they will launch a more in-depth security review following the Christmas Day scare. But one pilots' union has warned against splashing out on new technology while ignoring the basics, such as staff training.

"Fundamentally we should talk about the technology and the processes per se, but initially we should also talk about staff training," Georg Fongern, the deputy head of the German pilots' trade body, IFALPA, told German news agency DPA.

"It's crucial to ask what we can we expect from somebody carrying out the checks on minimal wages."