United Airlines, responding to hundreds of complaints last year, said today that it is buckling down on passengers who are so obese that they spill out of their seat and invade a fellow passenger's space.

The new policy requires that customers buy a second seat on a later flight "when they are unable to use an extended seat belt, put their armrests down, and if they infringe on another guest's seat," said Robin Urbanski, spokeswoman for the Chicago-based airline.

The change, which Urbanski said matches what other U.S. airlines are doing, is necessary for the "comfort and well-being of all our guests."

She cautioned, however, that the second-ticket purchase requirement will be enforced only after "all other solutions are exhausted, meaning the flight is full and we are unable to reaccommodate our guest next to an empty seat [on the initial flight] that is not occupied."

Should a second ticket need to be bought by the overweight passenger, it will be charged at the same price as the original, she said.

United fielded nearly 700 complaints in 2008 from passengers who said they had an uncomfortable flight because "the person next to them infringed on their seat, and this policy addresses this feedback," Urbanski said. The new policy applies to tickets bought on or after March 4 for travel on or after April 15.

For Delta Airlines, and its Twin Cities-based wholly owned subsidiary Northwest, the policy is much the same and has been for quite some time, said Delta spokesman Kent Landers. "If the plane is full, we offer the [overweight] passenger the option of purchasing an additional ticket on the next flight at the lowest fare cost available," Landers said.

Twin Cities-based Sun Country, however, said Wednesday that it has no policy addressing complaints about obese passengers. "It just doesn't happen very often," said Sun Country spokeswoman Wendy Blackshaw.

Southwest Airlines, the newest entry in the Twin Cities market, has for more than 25 years required any passenger who can't lower his or her armrest to buy two tickets so the passenger can occupy two adjacent seats. The flier is reimbursed if the flight is not sold out. "That happens 98 percent of the time," said Chris Mainz, spokesman for Southwest. The Southwest policy is so well-known, he said, that many obese customers routinely buy two tickets.

Staff writer Kerri Westenberg contributed to this report. Paul Walsh • 612-673-4482