Only 77.9 percent of May flights arrived within 15 minutes of schedule, down from 78.3 percent in the same month in 2006, the agency said Tuesday on its Web site. The complaint rate jumped to 1.13 per 100,000 passengers, compared with 0.78 last year.

Delays in May kept 2007's on-time arrival rate at the worst since 1995, a reflection of growing aviation gridlock as carriers add flights while the U.S. Federal Aviation Administration works to upgrade air traffic control equipment. Bad weather at some of the biggest hubs also slowed operations.

''It's off to a bad start, and travelers need to be prepared,'' said George Hamlin, managing director of Airline Capital Associates in Fairfax, Va.

The number of flights arriving on time has fallen each month in 2007, compared with a year earlier. For the year through May, the rate was 73.6 percent, the lowest annual figure since the U.S. began tracking the data in the current format 12 years ago, according to the Bureau of Transportation Statistics, a DOT agency.

The three New York-area airports - LaGuardia International, John F. Kennedy International and Newark Liberty International - again had the most delays, the BTS said. Flights to Oakland, Calif., were the most punctual.

The FAA is awaiting increased funding from Congress to speed upgrades of aging equipment that agency officials say isn't adequate for the traffic loads in U.S. airspace.

Travel in June began with delays, when thunderstorms and an FAA computer failure slowed air traffic at 11 major airports on June 8.

UAL Corp.'s United Airlines had its own computer malfunction on June 20, forcing it to stop takeoffs worldwide for more than two hours. Northwest Airlines Corp. cited weather, congested airspace and a pilot shortage for thousands of delays and cancellations during the last week of the month.

Continental Airlines Inc. said Monday that 67.9 percent of its planes arrived on time in June.