PARIS - Europe's Airbus said on Friday it was ordering immediate repairs to 6,000 of its widely used A320 family of jets in a sweeping recall affecting more than half the global fleet, threatening upheaval during the busiest travel weekend of the year in the United States and disruption worldwide.

The setback appears to be among the largest recalls affecting Airbus in its 55-year history and comes weeks after the A320 overtook the Boeing 737 as the most-delivered model. At the time Airbus issued its bulletin to the plane's more than 350 operators, some 3,000 A320-family jets were in the air.

The fix mainly involves reverting to earlier software and is relatively simple, but must be carried out before the planes can fly again, other than repositioning to repair centres, according to the bulletin to airlines seen by Reuters.

Airlines from the United States to South America, Europe, India and New Zealand warned the repairs could potentially cause flight delays or cancellations.

The world's largest A320 operator, American Airlines, said some 340 of its 480 A320 aircraft would need the fix. It said it mostly expected these to be completed by Saturday with about two hours required for each plane.

Other airlines said they would take planes briefly out of service to do the repairs, including Germany's Lufthansa, India's IndiGo and UK-based easyJet.

Colombian carrier Avianca said the recall affected more than 70 percent of its fleet, prompting it to close ticket sales for travel dates through Dec 8.

There are around 11,300 A320-family jets in operation, including 6,440 of the core A320 model, which first flew in 1987. Four of the world's 10 biggest A320-family operators are major US airlines: American Airlines, Delta Air Lines, JetBlue and United Airlines. European and Indian carriers are also among the jet's biggest customers.

For about two-thirds of the affected jets, the recall will result in a brief grounding as airlines revert to a previous software version, industry sources said.

ALSO READ: Airbus cuts 2,000 Defence and Space jobs, taming earlier plans

Still, that comes at a time when airline repair shops are already overrun by maintenance work, as hundreds of Airbus jets have been grounded due to long waiting times for separate engine repairs or inspections. The industry also has labour shortages.

"The timing is definitely not ideal for an issue like this to arise on one of the most ubiquitous aircraft around the (US) holidays," Mike Stengel of AeroDynamic Advisory said.

But due to the quick repair time, many jets can be fixed between scheduled flights or during overnight checks, he added.

Britain's easyJet said it had already completed the work.

A senior airline industry source, however, said sequencing the repairs at a time when demand is high and fleets are already facing maintenance delays was expected to be a major challenge.

Aviation analyst Rob Morris said there were questions over how much hangar capacity would immediately be available.

Airbus said a recent incident had revealed that solar flares may corrupt data critical to the functioning of flight controls.

Industry sources said the incident that triggered the unexpected repair action involved a JetBlue flight from Cancun, Mexico, to Newark, New Jersey, on October 30, in which several passengers were hurt following a sharp loss of altitude.

That A320 jet made an emergency landing at Tampa, Florida, after a flight control problem and a sudden uncommanded drop in altitude, prompting a Federal Aviation Administration investigation. JetBlue and the FAA had no comment on the recall.

The European Union Aviation Safety Agency late on Friday issued an emergency directive making the fix mandatory.

'Two hour repair'

An Airbus spokesperson estimated the repairs would affect 6,000 jets in total, confirming an earlier Reuters report.

The temporary groundings for repairs for some airlines could be much longer since more than 1,000 of the affected jets may also have to have hardware changed, industry sources said.

The abrupt recall sent ripples around the world.

A Finnair flight was delayed almost an hour as pilots established which software version they had, a passenger said. Air New Zealand warned of a number of cancellations.

Air France said it was cancelling 35 flights, 5 percent of the airline's daily total. Mexico's Volaris said it would be hit by delays or cancellations for up to 72 hours.

Launched in 1984, the A320 was the first mainstream jetliner to introduce "fly-by-wire" computer controls.

It competes with the Boeing 737 MAX, which suffered a lengthy worldwide grounding after fatal crashes in 2018 and 2019, blamed on poorly designed flight-control software.

Demand for the two main brands of workhorse jets has surged in recent years as economic growth led by Asia brought tens of millions of new travellers into the skies.

Originally designed to serve hubs, the single-aisle models were later widely adopted by low-cost carriers. The connections they provide now represent a significant slice of the economy.

The Airbus bulletin seen by Reuters traced the problem to a flight system called ELAC (Elevator and Aileron Computer), which sends commands from the pilot's side-stick to elevators at the rear. These in turn control the aircraft's pitch or nose angle.

READ MORE: Probe: Ruptured fuel hose behind Airbus engine failure

The computer's manufacturer, France's Thales, said in response to a Reuters query that the computer complies with Airbus specifications and the functionality in question is supported by software that is not under Thales' responsibility.