The Trump administration's plan to grow the Navy's fleet to 355 ships would cost an estimated $28.9 billion per year over the next 30 years, representing the largest naval buildup since the Reagan administration if completed, according to the Congressional Budget Office.

In a report released Thursday, the government watchdog said the expansion would cost the Navy 80 percent more annually than the average shipbuilding budget the service has received over the past three decades.

The CBO analyzed a Navy budget plan submitted to Congress in February for fiscal year 2019. The shipbuilding proposal details plans to purchase 245 combat ships and 56 combat logistics and support ships for a total of 301 new ships by 2048.

The CBO evaluation of the proposal found that the Navy's plan falls short of allowing the service to meet its goal of a 355-ship fleet at anytime in the next 30 years, unless the Navy significantly extends the service life of current warships.

Two months after submitting the shipbuilding plan, senior Navy officials told lawmakers that the service intends to extend the service life of all of its destroyers to 45 years, five to 10 years longer than indicated in the 2019 outline, and the life of seven attack submarines from 33 years to about 43 years. CBO said while the service life extensions would enable the Navy to reach a 355-ship fleet by 2034, it would fall short of service goals for specific ships.

The Navy's budget proposal also only accounts for the costs of new-ship construction, excluding other activities typically funded from the Navy's shipbuilding budget such as the cost of refueling nuclear-powered aircraft carriers or outfitting new ships with small pieces of equipment after they're built. CBO estimates that these costs would add $2.1 billion to the Navy's estimate of the annual shipbuilding budget.

The CBO report was published just days after the Trump administration announced the U.S. budget deficit ballooned to $779 billion in 2018, marking a 17 percent jump from the previous fiscal period.

President Trump on Wednesday called on all Cabinet secretaries to cut 5 percent from their budgets, though he suggested he would carve out an exception for the Pentagon.