The US Federal Aviation Authority (FAA) grounded all US flights for two hours today, after part of its air traffic control system failed.

Nearly 5,000 flights have been delayed so far and 868 were cancelled outright, after an overnight outage in the FAA’s Notice to Air Missions (NOTAM) system that provides safety information to flight crews. While flights are resuming, continued delays and congestion are expected.

At around 0715 EST (1215 GMT) this morning, the FAA ordered airlines to halt all domestic departures until 0900 (1400 GMT), while it restored the system and checked its integrity. The ban affected flights due to take off, but did not affect those in the air, which could still land safely.

Flights began again gradually from 0815, with the ground stop lifted at 0850

"Normal air traffic operations are resuming gradually across the United States following an overnight outage to the FAA’s Notice to Air Missions (NOTAM) system that provides safety information to flight crews," said an FAA statement at 0850. "The ground stop has been lifted."

NOTAM provides alerts on potential hazards along a flight route, and failed overnight, so flights did not begin as normal this morning. International flights into the US were also affected, with Air France and Air Canada specifically warning of delays.

The FAA has not issued any explanation of what caused the failure, but a spokesman for President Biden said there was no evidence that a cyberattack was to blame.

The President said earlier: "Aircraft can still land safely, just not take off right now," according to Sky News.

The NOTAM system dates back to 1947, when the United Nations determined that the developing air transport industry needed to share information about safety issues, in an agreed format, which includes a unique number,a description of the issue and a key word, along with the location and duration of the issue. Originally called "Notice to Airmen", the NOTAM service was modeled after Notice to Mariners used to advice shipping of navigation hazards.

Issues flagged by NOTAMs include temporary obstacles to runways, scheduled parachute jumps and air shows, and flights being made by heads of state.

NOTAMs have been criticized for being hard to decipher, leading to issues including a 2017 near-disaster when an Air Canada flight nearly landed on a runway that was closed to traffic and being used for taxiing aircraft. The change of use had been flagged in a NOTAM which the pilots did not register.

Since then, NOTAMs have been increasingly read by automated systems on planes.