The mistake, which was reportedly caused by a computer glitch, sold first class tickets from London to New York on the site for as low as $74 - a discount of more than $5,000.
According to Bloomberg, the airline will not honor the "several thousand" tickets sold during the glitch, which lasted several hours.
In a statement to Business Insider, United Airlines said:
United has temporarily suspended sales from its Denmark website because a third-party software provider applied an incorrect exchange rate and was misquoting the airline's properly filed fares. This error in the application of the exchange rate primarily impacted individuals who live outside of Denmark and who were not traveling to or from Denmark but were attempting to book tickets using United's Denmark site.