These airline ticket prices were never going to fly.

Thousands of airline passengers who bought transatlantic tickets for less than $100 have learned their tickets were too good to be true. 

United Airlines briefly offered online buyers deeply discounted first-class flights including London to Newark for just $74 US — only to void the sales once it realized the mistake.

United said in a statement Wednesday that the error was made by a third-party software provider that applied an incorrect currency exchange rate. 

"Most of these bookings were for travel originating in the United Kingdom, and the level of bookings made with Danish kroner as the local currency was significantly higher than normal during the limited period that customers made these bookings," the statement said.