All United Airlines flights were grounded for two hours on 20 June after a computer that weighs the aircraft before they take off suffered a glitch. Thousands of passengers were delayed. 

The computer ensures that the weight of the aircraft is properly distributed, calculates the quantity of fuel required for the flight and fine-tunes the flight plans just before take-off.

Its failure on Wednesday between 09.00 and 11.00 US Eastern Time delayed up to 250,000 passengers, according to calculations made by USA Today. 

United's busiest hubs, Chicago O'Hare and Denver, were worst affected with 24 US domestic flights cancelled and a further 268 internal and international flights delayed.

The airline was still suffering from knock-on delays today as it struggled to catch up. A notice on United's website warned passengers to check flights before arriving at the airport.

"We are very sorry for any inconvenience that Wednesday morning's computer outage may have caused you," the airline stated.

"While United's operation returns to normal, we continue to encourage you to check the status of your flight before going to the airport."

The incident follows a glitch in a flight-plan computer used by the Federal Aviation Administration which caused cancellations and delays US-wide on 8 June. 

United Airlines had the industry's highest rate of passenger complaints to the US Department of Transportation for all of 2006, according to USA Today. 

The airline clocked 1.36 complaints for every 100,000 passengers boarded, the worst since 2002 when United filed for Chapter 11 bankruptcy protection from which it emerged in 2006.