Some web users started getting access to United.com at around 5.45pm Eastern. The problem is likely to take hours to revert back to business-as-usual as, similar to the Qantas outage earlier this year, passengers are being issued hand-written paper boarding passes.
The total outage time was said to be two hours.
United Airlines lost all ability to transact online, in person and over the phone following a reservation systems failure earlier today.
At approximately 1:15pm eastern time in the US, United passengers started to report issues with United's web site, including ability to buy tickets and view previous reservations.
Soon after, reports started circulating on Twitter and other forums that passengers were unable to board their flights due to the systems failure. As of 4:45pm EDT, the system appears to remain offline and passengers are being advised to call back later.
United's web site currently displays a simple white page advising passengers that they are "experiencing technical difficulties".
Following its merger with Continental Airlines, United transitioned from its Travelport-operated reservation and hosting system to the HP-backed SHARES system.
The March transition caused a wide variety of issues which still plague United months after the transition, including delays in the posting of frequent flier miles, failure to process automated upgrades, and gate agent frustration with the command-line interface.
While United has promised improvements to the system later this year, a worldwide outage is sure to cause further questions whether the transition was a good idea.
The United outage comes after an awkward 2012 for airlines with their hosting arrangements. In January this year, Amadeus was blamed for a major failure of its ALTEA system, leaving a string of major airlines without services for hours (this followed a smaller incident just a few months before).
In July, Amazon hosted systems such as Room 77, Qantas (again) and Virgin Australia collapsed after a storm hit its hosting center in the US.