May 15--The fares were -- as advertised -- "out of this world." 

United Airlines unintentionally sold round-trip tickets for less than $25 on its website for about 45 minutes Tuesday, the same day it ran full-page ads touting a summer fare sale in Denver and Chicago. 

United said it will honor the tickets, which were priced on its website in error by a company that distributes fares for all airlines. United does not know how many customers bought the tickets, spokeswoman Chris Nardella said. 

"We discovered the problem and we fixed it, but there was a 45-minute window when customers were able to book these tickets. After that the site was down for about 30 minutes," she said. 

A United customer who contacted The Denver Post said he purchased a round-trip ticket between Denver and Boston for $23.50 on the website. He said customer-service representatives told him the airline would not honor the fare, but Nardella said that is not true. 

Customers who contacted United on Tuesday said they purchased tickets for $15 to $25, she said. 

The problem occurred when ATP Co., a clearinghouse for all airlines' fares, loaded new sale fares onto United's website, Nardella said. 

Initially, the web fares did not include a $5 discount that was supposed to be given to online customers. Then, when ATP tried to fix the problem, it posted the wrong fares, she said. Prices included taxes, facility charges and a $5 surcharge, but not the actual airline fares. 

It wasn't the first time United sold tickets so cheaply on its website. In January, 142 passengers bought tickets to international destinations for as little as $25. 

United first said it would not honor those fares, also the result of a technical problem, but later agreed to do so. 

United's current fare sale is for flights from its top hubs in Chicago and Denver. 

It lasts until May 24, for travel through Sept. 30. 

The fares, which require 14-day advance purchase, range from $202 round trip to Chicago to $378 to Miami and $874 to London. 

Frontier Airlines matched the fares on matching routes, though in some cases the Denver carrier's fares were already lower, spokeswoman Tracey Kelly said. 

Fare watchers said Tuesday that United's sale fares are about average for comparable summer sales going on nationwide. 

Overall, leisure fares are down from last year while carriers battle to fill seats as they restore flight schedules to pre-Sept. 11 levels. 

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