Major airline grounds entire fleet sparking travel chaos across the US

United Airlines has issued an urgent ground stop across its entire mainline fleet due to an active fire alarm at the integrated operations center.

The decision is expected to spark delays and travel chaos across the United States.

Passengers who are already in the air have been warned to 'expect some airborne holding due to excessive demand.'

It is unclear how long the ground stop will remain in place.

But it comes just days after Alaska Airlines issued a similar ground stop which sparked hours of chaos on Sunday night.

The major airline issued an extraordinary request to the US Federal Aviation Administration to stop all mainline flights across the United States due to a system-wide IT outage.

Planes were grounded from around 8pm PT (11pm ET) before operations resumed again at 11pm PT (2am ET), Alaska Airlines said in a statement.

The firm has as many as 238 planes in its fleet - all of which are Boeing aircrafts - and its biggest hubs include Seattle, Portland, LA and San Francisco. 

United Airlines has issued an urgent ground stop across its entire mainline fleet due to an active fire alarm at the integrated operations center 

Shortly after the initial request, a secondary order came through to extend the ground stop to subsidiary airline Horizon Air, which operates another 45 planes.

The ground stop was initially due to last until midnight, but was extended several times as authorities grappled with the IT problem.

The order was finally lifted three hours later - despite warnings of a moderate chance it would be extended once again. 

Alaska Airlines admitted the ground stop would cause delays for travelers and have knock on effects into Monday.

'At approximately 8 p.m. Pacific on Sunday, Alaska Airlines experienced an IT outage that's impacting our operations,' the airline said in a statement Sunday evening. 

'We requested a temporary, system-wide ground stop for Alaska and Horizon Air flights until the issue is resolved.' 

The airline has warned of 'residual impacts to our operation throughout the evening' amid concerns about widespread travel delays.

'If you are traveling tonight, please check the status of your flight before leaving for the airport.

The ground stop does not impact flights that are already in the air, but it does prevent any flights scheduled to take off within that window from doing so, sparking potential travel chaos with delays and queues

Alaska Airlines has issued a statement apologizing for the delay and explaining the reason for the ground stop

'We apologize to our guests for this inconvenience.' 

One passenger revealed he and his fiancé spent two hours waiting on a tarmac in Denver before the flight crew finally instructed them that the plane would not be taking off.

At 10.30pm, all passengers were instructed to disembark and await further instruction, he said.  

Another passenger, Riley Davis, said after two and a half hours on the tarmac, he still had no indication of when he would be free to deplane.

'Since they cannot coordinate exact gates, they are manually assigning gates via radio communication,' he revealed.

'I would love to know when I can get some dinner, I’m starving.'

He was inundated with comments from other travelers who said they'd noticed tarmacs around the country blocked by Alaska Airlines planes which had nowhere to go.