Southwest Airlines cancels 1,000 flights on Sunday and blames it on air traffic control issues and the weather: Passengers are left stranded across the country for a second day in a row after airline's pilots' union sued over vaccine mandate

Southwest Airlines has been hit by a second day of mass-flight cancellations, two days after the firm's pilots' union asked a court to block a COVID vaccine mandate.

The Dallas-based carrier canceled over 1,000 flights Sunday, equivalent to 27 per cent of its entire schedule for the day.

That came hot on the heels of 808 flights canceled on Saturday. Southwest has blamed a combination of inclement weather and staffing shortages at Jacksonville Air Traffic Control Center in Florida for the disruption.

But social media users have cited a Friday court filing by the airline's pilots union, the Southwest Airlines Pilots Association, as evidence of a different cause.

That filing sought to block pilots from having to have a COVID-19 vaccine, with Southwest setting a December 8 vaccination deadline.

It is federally-required to vaccinate its employees as a holder of US government contracts.