The transport secretary explained that a router that was being worked on had started flooding other air traffic control systems and created communication issues with planes, prompting the shutdown on safety grounds.

The government was now trying to understand whether the root cause was the wire replacement or a fault with the router.

The four-hour suspension from 15:54 EST (19:54 GMT) on Friday was the longest such incident since the 11 September 2001 attacks on the US.

More than 15,000 flights in the US were delayed or cancelled that day, according to data from flight tracking sites, affecting an estimated 1.5 million passengers, with more than 5,900 cancelled and 7,000 delayed on Saturday.

In some cases, passengers were left stuck inside planes on the tarmac, unable to take off or to return to the gates. The ground stop also had a knock-on effect on schedules around the world, with some flights out of the UK and Europe cancelled.

The Federal Aviation Administration (FAA) said the US flight schedule should return to normal by the end of Sunday.

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