Britain has authorised airport security staff to require any traveller to turn on electronic devices as a way to test for hidden explosives.
France and Germany are implementing similar measures, but only for travel to and from the United States.
American intelligence officials are worried about new al Qaeda efforts to produce a bomb. Powering up computers or phones allows screeners to make sure that explosives have not been substituted for batteries or essential components.
Travellers to and from Britain faced the new rules from Wednesday.
Travellers from France to the United States began undergoing checks Tuesday and the French measures were expected to last through the summer.
Germany says travellers to the United States should expect checks, but did not specify when they would start.
Security experts were sceptical. Michael Boyd, who heads aviation consultancy Boyd Group International, said would-be terrorists will find ways to make it look as if the device is working, for example by calling up a family photo.
"If you get good intelligence," he said, "you don't tell the other side you have it."