The aging attack submarines built during the Cold War are retiring faster than new submarines are being built to take their place.

China has the fastest-growing navy in the world today. Iran launched four new submarines last year. Yet the U.S. Navy cannot start buying more submarines from Electric Boat because the House and Senate cannot agree on a federal budget.

"Wherever you look you see the possibility of the U.S. being outnumbered around the globe and, therefore, the danger of us losing our current advantage," said Loren B. Thompson, a defense analyst and chief operating officer at the nonprofit think tank Lexington Institute.

The House and Senate passed another continuing resolution last week, which keeps funding frozen at last year's levels for another three weeks.

At this rate, the number of attack submarines could eventually drop below the 48 vessels the Navy says it must have to cover all areas of interest around the world, potentially to as low as 40 submarines, Thompson said.

Last year the Navy bought one Virginia-class submarine from Electric Boat for $1.96 billion. This year it is supposed to buy two for $3.44 billion.

"If we don't fully fund two subs a year before the end of this year, then obviously it's a math equation," said U.S. Rep. Joe Courtney, D-2nd District. "You end up with one sub, and that spills over to the following year."

The end result, Courtney said, could be a smaller fleet of submarines and "downstream that would affect our naval superiority."

Lt. Courtney L. Hillson, a Navy spokeswoman, said, "It would be fair to say that the budget impasse does put the Navy's most successful shipbuilding program at risk."

"The continued health of the Virginia program will directly support the Navy's continued ability to meet worldwide demands of combatant commanders," she said.

The Navy has modified its contract with Electric Boat twice since it was supposed to pay for the two fiscal 2011 submarines in full by Jan. 31. The service planned to pay for the first submarine and delay the deadline for fully funding the second submarine until today.

Courtney's office was told Friday that the deadline was extended again, to April 21, and $30 million in advance procurement funding will be used to buy parts so construction on the second submarine can start on schedule in the fall.

Both Courtney and Thompson expect the full funding for the second Virginia-class submarine to be allocated this year. But Thompson was not optimistic that next year's budget process would go any more smoothly than it has this year.

"There is no consensus on how to deal with the nation's deficit," Thompson said. "We may find ourselves back in another period of uncertainty within months."

That pattern could "persist for years," he said, jeopardizing the second submarine planned for next year or the year after, and progress on the program to replace the current fleet of Ohio-class, or Trident, submarines.

"This is uncharted territory for all of us," Robert Hamilton, spokesman for EB, said Friday. "We're working closely with the Navy to make sure we can get through this."

Hamilton referred questions to the Navy. The Navy has said that after March the current spending levels "will not support needed progress" on the Virginia-class program, and would jeopardize design work and construction start dates for the Ohio-class replacement, driving up costs by the delay.

j.mcdermott@theday.com