MPs are banned from wearing smartwatches during Cabinet meetings over fears that the devices could be hacked and used to listen in on government conversations.
Smartphones and tablet computers were banned in 2013 over security fears, but now smartwatches, including the Apple Watches worn by some Cabinet ministers, have been added to the banned list.
Number 10 was worried about the security services of China, Russia, Iran and Pakistan hacking connected devices and using them for surveillance.
But the focus is now very much on Russia. One source, quoted but not named by The Daily Telegraph, said: "The Russians are trying to hack everything."
In response to the supposed threat, Cabinet ministers have been required to hand their smartphones to security staff, who lock them in sound-proofed, lead-lined boxes before Cabinet meetings.
The first Apple Watches contained a number of high-profile security flaws, which ought to have been fixed with updates and new versions of Apple's watchOS operating system.
It was also feared that complimentary USB sticks handed out to delegates at the G20 Summit in Saint Petersburg in 2013 might contain a virus.
The Apple Watch was released in 2015, and a number of government ministers wore theirs in Cabinet meetings. Now, they too have been added to the banned list.
News of the Apple Watch ban comes after Russian spies were accused of trying to hack computers and devices that will be used in the US presidential election.
FBI director James Comey said that hackers based in Russia attempted to gain access to voter registration databases this year, and there are fears that certain makes of voting machine could be targeted.
However, a more basic explanation could be the distraction caused by smartphones and other devices. Former prime minster David Cameron reportedly banned BlackBerrys and other mobile phones in 2010 (when they were still widely used) for that reason.
Ex-education secretary Michael Gove, meanwhile, claims that he treated a Cabinet meeting to an accidental blast of Beyoncé while fiddling with his Apple Watch in 2015 before they were banned.
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