Major U.S. airlines are on track for their worst annual record for on-time performance ever with nearly a third of all flights delayed in June, government figures showed on Monday.

The 68.1 percent on-time arrival rate was the ninth-worst month on record and drove carrier performance for the first half of 2007 to its lowest mid-point mark -- 72.65 percent -- since the U.S. Transportation Department began keeping records in 1995.

More than 909,000 flights were delayed at least 15 minutes between January and June and more than 93,000 were canceled. It was the second highest cancellation rate since the first half of 2000, which is considered worst year so far for overall airline service.

Consumer complaints about lost luggage, late flights and other travel-related problems rose nearly 50 percent at 6,100 for the 6-month period.

Government and industry officials blame poor weather for most airline delays, which tend to peak in winter and the busy summer travel period.

But consumer and other industry experts say carriers have been saturating the system in recent years, leaving less and less recovery margin for unexpected events, like thunderstorms, aircraft mechanical problems or security delays.

The 20 airlines reporting data flew a record 3.69 million flights between January and June. In June alone, there were 628,000 scheduled flights, one of the highest monthly totals.

Airlines, however, are struggling with higher fuel costs and softening domestic demand to cement their modest financial recovery. They have no plans for wholesale changes to their operations and defend their scheduling practices.

Trying to maximize their yields, carriers have pared seat availability this year already and planes are flying at or near capacity.

Airplanes are flying when people want to fly and where they want to fly, said David Castelveter, a spokesman for the industry lobbying group, the Air Transport Association.

Airlines say there is little that can be done in the short term to reduce delays. We're working with the Federal Aviation Administration to find some solutions, he said.

One concentration point is New York's John F. Kennedy International Airport, where only half of all flights were on time in June. JFK is traditionally a connecting point for international flights but has become a growing alternative to LaGuardia and Newark airports for domestic service.

JetBlue Airways Corp is based at JFK while Delta Air Lines has major operations there. JFK accounted for a significant number of the longest departure delays last month, some of which involved JetBlue flights, records show.

Air traffic management does need to improve, said JetBlue spokesman Bryan Baldwin. He noted that the airline's reliance on one airport for 360 of the carrier's 550 daily flights maximizes chances that its operations will be disrupted when problems arise. A February ice storm in New York disrupted JetBlue for days.

One possible solution, the airline has said, is for the FAA to check growth by limiting the number of daily flights at JFK.

Michael Boyd, a Colorado-based industry consultant, disagrees with flight caps and said the airlines are being penalized for years of meaningful government inaction on air traffic system improvements.

The problem is not new, Boyd said. We've got to fix the air traffic control system and that fix is not anywhere near being done.