DENVER (AP) _ Three airlines are raising ticket prices by up to $40 a round trip for travelers using the new Denver International Airport, whose opening is at hand after more than a year's worth of delays.

United, American and Frontier airlines announced the fees over the weekend, saying they will apply only to passengers whose flights originate or terminate in Denver _ not to passengers making connections here.

The fees will take effect when the $4.2 billion airport opens. Mayor Wellington Webb is expected to announce this week that Denver International will open Feb. 28. The opening has been delayed four times over 16 months caused, chiefly by problems with the airport's baggage handling system.

United, the airport's biggest user, and American will charge an additional $20 to passengers arriving or departing from Denver _ $40 for a round trip. Frontier said it will charge an additional $30 for a round-trip ticket.

United said it has to charge passengers extra to help meet the airline's estimated $195 million in annual rents and fees at Denver International, more than five times as much as it pays at Stapleton International Airport. There's also the expense of building a maintenance hangar and other facilities.

Webb said he was disappointed with United.

``This is not what we expect from a corporate citizen such as United,'' said Webb in a statement addressed to United Chairman Gerald Greenwald.

``(But) we have come too far at this point, as we near the opening of DIA, to see actions such as this infringe upon our spirit of friendship,'' he wrote.

United has not said how much the increase will generate in additional annual revenues this year, but the airline is expected to move about 20 million passengers through Denver in 1995.

United sales manager Tom Healy said the fare increase is a move that ``any reasonable business makes when its costs increase.''

Diane Koller, the city's deputy aviation manager, said United must bear part of the responsibility for some of the airport's high costs.

``United made decisions about how the facility should look and operate and that made it a lot more costly,'' Koller said. ``A lot of the things they wanted drove up the cost for everyone.''