A new $450 million AirTrain to LaGuardia Airport from the Mets-Willets Point stop on the jam-packed No. 7 line wouldn't be worth the trip, transit advocates warned Wednesday.

"To put a new group of airport-bound travelers and their luggage on the already-overcrowded 7 line for 20 stops would be a disaster," said Andrew Albert, chairman of the Transit Riders Council and MTA board member. "You think the 7 is crowded now? Just wait."

The Mets-Willets Point stop would be the 20 station on the line if a rider boards at the new extension on 34th Street and 11th Avenue.

The MTA is modernizing the No. 7 train's signal system to run more trains in booming Queens, but the line is at 100% capacity during rush-hour.

Albert said it would be faster to take the existing Q70 express bus to the airport, which runs from Jackson Heights and Woodside. It could be accessed without the No. 7 line by taking the E, F, and R to Jackson Heights, or the LIRR to Woodside. The M is available on weekdays.

State officials said the Willets Point connection was chosen because it is the location least likely to disrupt nearby neighborhoods.

The rail link, which Gov. Andrew Cuomo proposed earlier this year, is part of a massive plan to tear down LaGuardia and replace it with a modern airport.

"The governor's proposal requires additional planning and study on the part of the MTA and the Port Authority, and most importantly, the community will be involved throughout that process," said Cuomo spokeswoman Beth DeFalco.

"It makes absolutely no sense for LaGuardia to be the only airport in the region to not have a public railway."

Construction on the new airport is expected to start next year, and most facilities on the site to be done within three years.