WASHINGTON — A government watchdog is warning that pushing too fast toward construction with unproven and immature technologies could drive up the price of and delay the Columbia-class nuclear missile boats — the U.S. Navy’s No. 1 budgetary priority.

The Government Accountability Office said the Navy is overselling how far along some crucial technologies are, including its reactor, the integrated power system, propulsion system and the U.S.-U.K. common missile compartment. The Navy has moved into detailed design with immature technologies, which has driven up costs and created delays in other programs, the GAO found.

“Proceeding into detail design and construction with immature technologies can lead to design instability and cause construction delays,” the report reads. “The Navy plans to accelerate construction of the lead submarine to compensate for an aggressive schedule, which may lead to future delays if the technologies are not fully mature before construction starts, planned for 2021.”

But the Navy and the Department of Defense disagree, arguing among other things that the GAO uses too high a standard for judging when a technology is considered mature.

”[The GAO’s] approach would require all technologies on a shipbuilding program to be prototyped at full-scale and demonstrated in at-sea environments — essentially building a full-sized prototype submarine — before authorizing the lead ship,” James MacStravic, the acting assistant secretary of defense for acquisition, wrote in the DoD’s response.

That approach would greatly increase the cost of the program and would delay the launch of the lead ship, presenting a dangerous interruption in U.S. strategic deterrence policy.

The Navy aims to have the first ship on a nuclear deterrent patrol by 2031, a timeline it must meet as the aging Ohio-class submarines reach the end of their service lives. The 12-ship Columbia-class program is designed to replace 14 of the Ohio-class boomers, and the with the first patrol just 13 years away, the Navy has said there is no room for error.

The Ohio-class subs will be 42 years old when they leave the fleet. In September, General Dynamics Electric Boat received a $5.1 billion contract for detailed design of the class.

With a tight schedule in mind, the Navy has been forced to forge ahead with detailed design despite risks of cost and schedule delays, said Thomas Callender, a retired submariner and analyst with the Heritage Foundation.

“We can’t push off any further, even if there is some risk as we push forward,” Callender said. “We don’t have any choice, and I think everyone knows the stakes here. There has and will be a lot of attention to issues early so they’ll be overcome, but in this case schedule has to override cost risks in this lead ship.”

The Navy has been finding ways to drive down costs and technology risks by using best practices and designs from the Virginia-class attack submarines, but with new technologies there are always risks, Callendar said.