Major U.S. airlines negotiating insurance renewals during the fourth quarter face a market that has hardened significantly since a year ago, brokers say.

Airlines have faced average rate increases between 10% and 20% for hull and liability insurance so far this year, a trend that may worsen in the fourth quarter. Most major airlines renew their insurance programs in November and December, including Continental Airlines Inc., Delta Air Lines Inc., JetBlue Airways Corp., Southwest Airlines Co., United Airlines, US Airways Group Inc. and United Parcel Service Inc.

According to various broker estimates, 60% to 80% of the market renews during the fourth quarter.

“We don’t expect a massive change in the trend between now and the end of year,” said Steve Doyle, London-based executive director at Willis Aviation, a division of broker Willis Group Holdings Ltd. “The trend is likely set for the early part of 2010.”

About 78% of placements so far this year have faced increases in lead hull and liability costs, according to Aon Ltd. In 2006, only 22% of renewing accounts faced such increases.

Brokers said following insurers have been particularly aggressive. The traditional disparity between the rates for lead and following insurers has dwindled to low single digits, Mr. Doyle said.