WASHINGTON (Reuters) - Expanding the U.S. Navy to 355 ships as recommended by military leaders and backed by President Donald Trump would cost some $400 billion more over the next 30 years than the currently planned 308-ship fleet, according to a study released on Monday.

FILE PHOTO - Sailors man the rails of the USS Carl Vinson, a Nimitz-class aircraft carrier, as it departs its home port in San Diego, California August 22, 2014. REUTERS/Mike Blake/File Photo

The annual cost to build, crew and operate a 355-ship fleet would be about $102 billion, or 13 percent more than the $90 billion needed for the currently planned Navy, according to the study by the nonpartisan Congressional Budget Office.

The $102 billion cost of the 355-ship fleet is 33 percent more than Congress appropriated in 2016 for the current 275-ship Navy, the CBO said.

To achieve the larger force, the Navy would need $26.6 billion annually for ship construction, which is 60 percent more than the average amount that Congress has appropriated for shipbuilding in the past 30 years, the CBO study said.

The Navy’s 2017 shipbuilding plan called for boosting the size of the fleet to 308 ships, which was expected to cost $21.2 billion per year to implement over 30 years.

With Trump pressing for an expansion of the fleet to 350 ships during the presidential campaign last year, the Navy released a new force structure assessment in December seeking a 355-ship Navy.

Taking into consideration older ships being retired, creating a 355-ship fleet would require the Navy to buy about 329 new ships over 30 years, compared with 254 under its previous plan for a 308-ship fleet, the CBO study found. The Navy would have to buy about 12 ships per year under the larger fleet plan, versus about eight per year under the earlier plan.

The larger fleet would require more civilian and uniformed personnel and more aircraft, pushing up overall operating costs, the CBO said.

The increase in shipbuilding would force all seven U.S. shipyards to expand their work forces and improve their infrastructure in order to meet the demand for vessels, the CBO said. The greatest challenge would be building submarines to meet the force structure requirements, the report said.

The study said the earliest the Navy could achieve a 355-ship fleet would be the year 2035, or 18 years from now.