Passengers on flights to the UK from the Middle East and North Africa face a ban on laptops and iPads amid fears over new terrorist tactics.
Tougher rules hitting thousands of holidaymakers and business travellers every day mean electronic devices larger than a phone have to be stored in the hold.
Baffled computer experts questioned the design of the new rules, suggesting a laptop bomb would be just as effective in the hold as in the cabin.
The move to beef up UK security - which comes into force immediately - will cover all direct flights from Turkey, Lebanon, Jordan, Egypt, Tunisia, and Saudi Arabia.
The ban covers all devices bigger than a smart phone - meaning even iPad mini and Kindles are excluded, alongside full size laptops.
The Prime Minister's spokesman said: 'Safety of the travelling public is our highest priority. That is why we keep aviation security under constant review and put in place measures that are necessary, effective and proportionate.
'Over the last few weeks, PM has chaired a number of meetings on aviation security, most recently this morning, where it was agreed new measures will be introduced.'
The shake-up comes after America imposed similar but wider restrictions on flights from eight countries.
UK security sources have seen the same intelligence as their US counterparts and are responding to the perceived threat.
The arrangements have been in preparation for weeks, and are broadly similar to those introduced by America but not identical.
The move is not thought to be in response to a specific plot.
The rules will cover all kit larger than 6.3 inches long, 3.6 inches wide and 0.6 inches deep. This covers all major smartphones but is smaller than an iPad mini or a Kindle.
The No10 spokesman said: 'Under the new arrangements, passengers boarding flights to the UK from the countries affected will not be allowed to take any phones, laptops or tablets larger than a normal sized mobile or smartphone into the cabin of the plane.
'Any such devices will need to be placed into hold luggage and checked-in before going through central security. Normal cabin baggage restrictions will continue to apply.
'The additional security measures may cause some disruption for passengers and flights, and we understand the frustration that will cause, but our top priority will always be to maintain the safety of British nationals.
'Direct flights to the UK from these destinations can continue to operate to the UK subject to these new measures being in place.'
Transport Secretary Chris Grayling said the Government was not currently advising against travel to the included countries.
He said: 'We understand the frustration that these measures may cause and we are working with the aviation industry to minimise any impact.
'Our top priority will always be to maintain the safety of British nationals.
'These new measures apply to flights into the UK and we are not currently advising against flying to and from those countries.'
He added: 'We remain open for business. People should continue to fly and comply with security procedures.'
Nicholas Weaver, researcher at the International Computer Science Institute at the University of California, Berkeley, told the Guardian: 'It's weird, because it doesn't match a conventional threat model.
'If you assume the attacker is interested in turning a laptop into a bomb, it would work just as well in the cargo hold.
'If you're worried about hacking, a cellphone is a computer.'
The Emirates airline confirmed today it was implementing restrictions following a security directive from the US Transportation Security Administration.
The US Department for Homeland Security said the procedures would 'remain in place until the threat changes', and did not rule out extending them.
The US version restrictions apply to airlines flying direct to the United States from airports in Jordan, Egypt, Turkey, Saudi Arabia, United Arab Emirates, Kuwait, Morocco and Qatar.
The airports affected are in Amman, Cairo, Kuwait City, Doha, Dubai, Istanbul, Abu Dhabi, Casablanca, Morocco, Riyadh and Jeddah, Saudi Arabia.
Concerns were raised last year when insurgent group al-Shabaab smuggled an explosive-filled laptop on to a flight out of Mogadishu.
It blew a hole in the side of the plane, but the aircraft was still low enough that the pilot was able to land the plane safely.
The tougher rules come weeks after it was revealed that UK security services have foiled 13 potential attacks in less than four years, while counter-terrorism units are running more than 500 investigations at any time.
The official threat level for international terrorism has stood at severe, meaning an attack is 'highly likely', for more than two years.
Liberal Democrat transport spokeswoman Baroness Randerson said countries seem to have been 'singled out based on religious beliefs'.
'Of course safety is paramount at all times and we all need to remain vigilant but this ban needs to be explained in detail,' she said.
'Serious questions remain unanswered. Like why the Government think it will work and why are these originating countries are being singled out based on religious beliefs?
'We have seen time and again that terrorism comes from some of the unlikeliest sources.'
Air industry consultant John Strickland warned that the ban will cause 'headaches for airlines and customers' but said carriers have 'no choice but to put security first' when official advice is given.
He added: 'One unexpected consequence will be the challenge of additional devices with lithium batteries being stowed in the holds which brings its own challenges to safety.'
Travel trade organisation Abta warned that laptops and tablets are not typically covered by travel insurance policies for loss, damage or theft if they are placed in the hold.
A spokeswoman for the organisation said: 'Passengers travelling to the countries affected may wish to consider leaving their electronic devices at home, although this may be difficult for many, especially business travellers and families travelling with children.'
The airlines facing disruption from new UK restrictions:-
- British Airways
- Thomas Cook
- Turkish Airlines
- Pegasus Airways
- Atlas-Global Airlines
- Middle East Airlines
- Royal Jordanian
- Tunis Air
© Associated Newspapers Ltd.