The bill, which President Bush is expected to sign, allows the shipbuilders to pay 40 percent of their estimated income tax during the life of the ship contract and the remaining 60 percent in the year that construction is completed. Shipyards now estimate profits during each year of construction of a ship and pay income taxes on those estimates.

While critics call the bill a pork-barrel measure that benefits only a few large corporations, its backers say it provides greater fairness by assuring that shipbuilders won't be taxed on profits they haven't yet earned.

The provision's supporters say that over time, the change won't cost the government anything.

"It is a matter of cash flow; it doesn't reduce their taxation," Snowe, R-Maine, said Wednesday. She noted that the entire corporate-tax bill is revenue-neutral, and that shortfalls created by provisions like the shipbuilding tax adjustment have been offset by closing tax shelters and other corporate loopholes,

While shipyards will pay the same taxes in the long run, they will benefit by being able to use money normally paid in taxes to invest in their business, or to make money in other ways.

"We haven't had a real hard look at it," said Kendell Pease, spokesman for General Dynamics, BIW's parent company. "It's being examined to see what real impact we'll have."

Critics of the provision say past experience suggests that the tax deferral could be abused.

"It's technically a deferral; the problem is it's a deferral that just keeps deferring," said Robert S. McIntyre, director of Citizens for Tax Justice, a Washington-based tax watchdog group. "Year in and year out, they never seem to pay anything."

Snowe's office said that the provision's estimated cost of $500 million was a transitional figure representing the drop in tax revenues that would occur while the systems changed over. In the long-run, the $500 million would be recouped.

Snowe said the change was also giving naval shipyards the same advantages already enjoyed by their commercial counterparts.