The US is barring passengers on some international flights from mostly Middle Eastern and North African countries from taking laptops, tablets, electronic games and other devices on board in carry-on bags.
Passengers flying direct to the US from 10 airports in eight countries will be allowed only mobile phones in passenger cabins, senior Trump administration officials said. Larger electronic items must be checked in.
The rules took effect on Tuesday, and airlines have until Saturday to implement them or face being barred from flying to the US, officials said.
They said the decision was prompted by "evaluated intelligence" about potential threats to planes bound for the US. The officials would not discuss the timing of the intelligence or if any particular terror group is thought to be planning an attack.
The electronics ban affects flights from international airports to the US from Amman, Jordan; Kuwait City, Kuwait; Cairo, Egypt; Istanbul, Turkey; Jeddah and Riyadh, Saudi Arabia; Casablanca, Morocco; Doha, Qatar; and Dubai and Abu Dhabi in the United Arab Emirates.
About 50 flights a day, all on foreign carriers, will be affected. The officials said no US-based airlines have non-stop flights from those cities to the US.
The UK later followed suit, saying flights from six countries will have a bar on passengers taking "any phones, laptops or tablets larger than a normal-sized mobile or smartphone", into the cabin. The UK rules will apply to flights from Turkey, Lebanon, Jordan, Egypt, Tunisia and Saudi Arabia.
With the US order affecting flights from predominantly Muslim nations, the ban may invite comparisons to Donald Trump's orders barring travel from several Muslim-majority-nations, which have been blocked by courts.
Early in his candidacy, Mr Trump called for the barring of Muslims from entering the US.
The travel ban was more severe, separating families and barring students from studying in the US. The laptop ban is more of an inconvenience and its stated reason is to protect the travellers who are affected by it.
It is still likely to annoy powerful business people and diplomats, and could affect the travel plans of wealthy tourists sought after by the US travel industry.
Details of the electronics ban were first disclosed by Royal Jordanian and the official news agency of Saudi Arabia.
In its statement, Royal Jordanian said the electronics ban would affect its flights to New York, Chicago, Detroit and Montreal.
A spokesman said the airline has not yet started to enforce the new regulation.
EgyptAir officials said the airline will implement the ban on Friday, while Emirates officials said the new procedures would start on Saturday.