Because of the error, United began selling thousands of ultra-cheap tickets Wednesday morning, including first-class transatlantic flights departing from England for as little as $51. Within a few hours the airline stopped those sales and issued a statement saying it would not honor those prices.
A U.S. Department of Transportation rule prohibits airlines from increasing the price of a ticket after a passenger has booked and paid for it. It is not clear if this incident is a violation of that rule. Travelers whose tickets were voided were asked to pay the accurate, more expensive fares to rebook those flights.
In a statement, the Department of Transportation said it has launched an investigation, based on complaints from travelers who bought the super-low fares.
The office of Aviation Enforcement and Proceedings "will gather all the relevant facts before making a determination as to whether United is or not required to honor the mistaken fares," the department said.
United declined to comment but issued a statement that blamed the problem on an error in converting the Danish krone to other currencies for flights originating in the United Kingdom.
This is not the first time an airline has sold deeply discounted tickets because of an error.
Delta Air Lines and United blamed either human error or computer glitches for the sale of tickets discounted as much as 90% off regular prices in 2013. Both airlines ultimately agreed to honor those fares.