The Federal Aviation Administration, which released the forecast Thursday, faces spending cuts for runways, air traffic control equipment and buildings. But the agency’s administrator, Marion Blakey, said she was

‘‘We are redesigning airspace, deploying new software that will help increase capacity, and putting new procedures in place,’’ Blakey said. ‘‘We will be ready.’’

Lawmakers and aviation advocates were not so sure.

Building is not keeping up with the increase in passengers, said David Stempler, president of the Air Travelers Association. ‘‘That just spells congestion and delays for passengers.’’

Already, flights have been limited at Chicago’s O’Hare International Airport because too many planes were trying to take off and land, causing delays throughout the country. The FAA negotiated an agreement with airlines to cut 37 daily flights and limit the number of domestic arrivals to 88 an hour between 7 a.m. and 8 p.m.

Ruth Marlin, executive vice president of the air traffic controllers union, said many passengers will do a lot of waiting in 2015 if things do not change.

‘‘The FAA is trying to do more and more with less and less and that is putting an incredible strain on the system,’’ she said.

Sen. Christopher Bond, chairman of the Senate Appropriations subcommittee that oversees transportation spending, has expressed disappointment in the Bush administration’s budget proposal for 2006. It would cut money for airport construction and runways by $500 million next year, to $3 billion.

‘‘I am at a loss to understand why this program remains in the sights of the budget gnomes,’’ Bond, R-Mo., said at a hearing this week.

Sen. Patty Murray, the top Democrat on the subcommittee, pointed out the administration has proposed $77 million in cuts for air traffic control modernization, in addition to $400 million cut this year. In 2004, the FAA was authorized to spend $2.9 billion.

‘‘All indications are that air traffic will continue to grow,’’ said Murray, D-Wash. ‘‘Yet the Bush administration has decided that now is the time to impose dramatic cuts in our investment at improving safety and expanding capacity at our airports.’’

David Plavin, president of the Airports Council International-North America, said the problem is not just the increase in passenger traffic, but that planes are getting smaller. Small planes place just as much a burden on the air traffic system as large planes.

‘‘FAA is chronically underfunded,’’ said Plavin, whose group represents airports. ‘‘Some air traffic control towers are chronically understaffed.’’

But Blakey said the dollars for airport runways and buildings would still be twice what it was in the late 1990s, when airports received about $1.5 billion. In September, she said, the FAA assessed airport authorities capital needs and found they were 15 percent lower than the year before.

The FAA, which forecast a 45 percent passenger increase by 2015, also said:

Traditional big airlines such as United and American will grow at a sluggish pace, with the number of passengers increasing by 2.8 percent annually over the next 12 years.

Regional carriers, which fly planes with 70 or fewer seats, and international travel will fuel the increase in commercial aviation.

The number of passengers on regional airlines will increase 15.4 percent next year.

The number of passengers flying to and from the United States on domestic airlines will increase an average of 5 percent annually over the next 12 years.