Voters approved a plan to let Denver annex land for a new $3 billion airport that would be the first major U.S. airport since 1974 and bring relief to one of the country's most congested hubs.

The vote from Adams County's 164 precincts in Tuesday's special election was 34,070 or 55.9 percent for annexation and 26,928 or 44.1 percent against.If the vote is certified by the county clerk and the secretary of state, 45 square miles of unincorporated county land 10 miles northeast of Stapleton International Airport will be annexed by Denver, probably within 10 days, officials said.

"This is the rocket booster that pushes the airport project forward," said a jubilant Denver Mayor Feder-ico Pena after the election.

Pena said city officials would direct their energies to finishing an environmental impact statement required by the Federal Aviation Administration before construction of the airport, which the FAA says could be the nation's second-busiest by the year 2000.

The FAA supported the proposed airport because Stapleton, the world's fifth-busiest airport, already is congested and is expected to lead the nation in flight delays by 1996.

Proponents, who devoted more than $1 million to the campaign, outspending foes by about 10-to-1, said the airport would boost Colorado's economy and provide jobs. They said development would mean property tax relief.

Opponents argued that closing Stapleton would be a waste of money. They were concerned about increased air pollution, jet noise over homes and the effects on the 2,700-acre Barr Lake State Park bird refuge near Brighton.

The new airport is the first planned in the United States since Dallas-Fort Worth International Airport opened in 1974.