WHO IS AFFECTED?
Travellers flying to the United States from 10 airports in eight Muslim-majority countries are affected.
The airports are in Cairo, Egypt; Istanbul, Turkey; Kuwait City, Kuwait; Doha, Qatar; Casablanca, Morocco; Amman, Jordan; Riyadh and Jeddah, Saudi Arabia; and Dubai and Abu Dhabi in the United Arab Emirates.
Passengers are prohibited from carrying all electronic devices larger than an average-sized mobile phone, including game consoles. These must all be checked in.
The British ban affects flights from Turkey, Lebanon, Egypt, Saudi Arabia, Jordan and Tunisia.
All phones, laptops and tablets larger than a normal-sized mobile or smartphone must be checked in.
Britain has specified that the ban would apply to devices bigger than 16cm in length, 9.3cm in width and 1.5cm thick, so some e-readers like Kindles will be affected.
Travellers at Changi Airport flying to the US or Britain on airlines that stop at the affected airports must ensure their laptops or other large electronic gadgets are checked in. Otherwise, they risk having their items confiscated.
Reacting quickly to allay travellers' concerns, Emirates said on Friday that they will be allowed to keep their laptops and other banned devices until just before boarding flights to the US.
At the gate, these items will need to be surrendered so they can be packed in boxes and carried in the plane's cargo hold during the flight.
In the wake of the US and British ban, Canada and France are considering imposing similar measures while in Singapore, the police and Civil Aviation Authority of Singapore said they are monitoring developments.
According to the International Air Transport Association, the current ban will impact 393 scheduled passenger flights a week to Britain - about 2.7 per cent of the total international scheduled passenger flights to the country. About 350 flights a week to the US - or 2 per cent of international flights to the country - will be affected.
WHY THE BAN?
This is the biggest question on travellers' minds, and the reason many cannot make sense of it is because nothing has happened - yet.
When airports started asking travellers to remove their shoes, many were, and continue to be, irritated, but at least they can understand why it is being done.
On Dec 22, 2001, Richard Colvin Reid, better known as the Shoe Bomber, attempted to detonate explosives packed into the shoes he was wearing while on American Airlines Flight 63 from Paris to Miami. He was subdued by passengers, arrested and indicted.
Then came the ban on liquids, gels and aerosols which, again, was prompted by an incident - this time when the British authorities foiled a plot in 2006 to bomb as many as 10 transatlantic airliners with liquid explosives.
It led to new rules that all liquids and gels must be stored in containers with a maximum capacity of 100ml each.
These containers must be packed in resealable transparent plastic bags with a maximum capacity not exceeding 1 litre, and be made available for visual examination.
That the requirement is now mandatory at all major airports has made it easier for travellers to accept that the threat must be grave indeed.
With the current ban on laptops and other electronic devices, there is little transparency, which security experts say cannot be helped, given the nature of the threat.
All that is known publicly is that the moves by the US and Britain reportedly came after a US raid on terrorist targets in Yemen in January found that terrorists have successfully developed compact battery bombs that fit inside laptops or other devices.
Believed to be strong enough to bring an aircraft down, the bombs need to be manually triggered.
What is not known: Was there a specific flight or flights targeted?
Were there plans to attack specific airports or cities?
Is the danger in the laptop being used as a bomb or in terrorists hiding potentially dangerous substances inside the laptop?
The other million-dollar question: Does the current ban mean that, all this while, travellers have been wasting their time removing laptops from bags and turning them on to prove the equipment is genuine?
An expert with The Soufan Group, a New York-based security and research consultancy, said a change of this magnitude suggests that the US is concerned that terrorist groups have made "some quantum leap in their usual modus operandi that current countermeasures won't detect".
Professor Rohan Gunaratna who heads Nanyang Technological University's International Centre for Political Violence and Terrorism Research, said it is important to understand that terrorists have become smarter in finding ways to circumvent security systems and processes.
"Security cannot be routine. The threat is dynamic, constantly evolving, and governments must act and take necessary measures to mitigate any risks that come up," he said.
Mr Jacques Astre, president of consultancy firm International Aviation Safety Solutions, said: "It sounds like they have intel about something very difficult to detect and that's why the restrictions are so draconian."
Mr Chris Bala, managing director of Singapore-based CJ Security Consulting which provides training and consultancy in aviation security, said: "Whatever you do, there is no such thing in aviation security as a 100 per cent foolproof plan, so (the ban) is to further minimise and reduce the potential of anything slipping through."
THE CONSPIRACY THEORY
All but one of six experts rubbished the idea that the ban is to clip the wings of key Middle Eastern carriers, dubbed super-connectors, for their significant transit traffic.
The US list includes airports in Dubai, home of Emirates; Doha, where Qatar Airways is headquartered; and Abu Dhabi, where Etihad Airways is based.
All three carriers are popular with travellers, including those from Singapore, who travel long haul to the US and Europe.
It is noteworthy that the three are not on Britain's watch list.
It is no secret that aggressive expansion by these airlines in the last decade has caused much grief for other long-haul premium carriers like Singapore Airlines, Cathay Pacific, Lufthansa and British Airways, as well as American carriers.
The state-owned Middle Eastern carriers, despite all that they have said to the contrary, have always been accused of benefiting unfairly from state subsidies.
But even as the ban will hurt them, it does not make sense for governments to resort to such tactics to tame the giants.
It would be much easier to just turn protectionist by denying them landing slots at airports, for example, or giving them unpopular landing and departure times to make their flights less popular.
In this case, experts noted that the three airports, along with others on the watch list, are located in and around areas where terrorist activities are present.
In particular, Dubai, Doha and Abu Dhabi - despite their world-class security equipment and processes - could be deemed vulnerable because they are major hubs connecting people from different countries. That the three are on the US list but not Britain's could just mean the US is less confident of security processes and systems at these airports.
Offering a different view, Mr Abdul Basit, an associate research fellow at the S. Rajaratnam School of International Studies (RSIS), noted that if security was the sole motivator, it would make more sense to install more sensitive scanners, for example.
He said: "It is puzzling why an electronic device is dangerous inside the cabin and not in the cargo...
"If someone manages to carry an explosive-laden electronic device inside the plane - whether in the cabin or the cargo - he can trigger it remotely."
The rule also makes no sense "because anyone trying to target an airline can board a flight from countries not affected by these restrictions", he said.
Such loopholes have forced people to question the ban's logic, purpose and objectives.
While they do not necessarily accept the conspiracy theory, other experts agree it does not make sense to limit the scope of the ban.
Associate Professor Kumar Ramakrishna, head of policy studies at RSIS, said: "If you just have rules covering some but not other international airports, the terrorist networks that have been targeting aviation will just exploit the loopholes with the help of supporters or cells elsewhere."
At some point, the new regulations for laptops should apply to all airports, as in the case of the regulations issued after the Shoe Bomber incident in 2001 and the London liquid explosives plot in 2006, he said.
"If you just focus on Muslim countries, it also strengthens the impression that the whole thing is anti-Islam, which will play into the hands of the ISIS (Islamic State in Iraq and Syria) narrative that the West is against Islam," he stressed.
WHAT CAN TRAVELLERS EXPECT FROM NOW?
Whether it is checking shoes or imposing restrictions on liquids and gels, history has shown that when it comes to aviation security, the US and Europe will lead, and the rest of the world follows.
In this case, much depends on how long the current ban will stay, experts say. If it becomes the new normal, other countries can be expected to follow suit.
There is no doubt the threat is real and grave, Prof Gunaratna said, with ISIS expanding beyond the heartlands of Iraq and Syria to become a global movement.
Perhaps more alarming is the group's growing capabilities.
He said: "ISIS is no longer just recruiting from the periphery of the Muslim community. They now appeal to mainstream Muslims as well, recruiting even scientists and other technical experts."
For governments and security agencies, the aim is always to be one step ahead and to engage the community in the war against terrorism.
As for travellers, there is really no choice but to comply with the regulations.
Prof Kumar said: "As passengers have adapted to the previous regulations in response to the evolving threat, they will ultimately do so in this case as well.
"This, unfortunately, is all part of the 'Osama bin Laden tax' that everyone has been paying to ensure that air travel safety is not compromised in the age of rapidly evolving threats to aviation," he added.