The HMS Vengeance, a Vanguard-class submarine, departs for Devonport, England, prior to refit February 27, 2012, off the coast of Largs, Scotland. Andrew Linnett/MoD Crown Copyright via Getty Images

LONDON (Reuters) -- The overall cost of replacing and maintaining Britain's nuclear deterrent will reach 167 billion pounds ($256 billion), much more than expected, according to a lawmaker's and Reuters' calculations based on official figures.

The Scottish Nationalist Party, which wants Britain's Scotland-based nuclear-armed Trident submarines scrapped, called the sum "unthinkable and indefensible" at a time when deep cuts under the government's "austerity" policies mean "thousands of people across the UK are struggling to afford basics like food."

Some military officials also oppose investment in Trident, saying the money would be better spent on maintaining the army and on more-conventional technology, which have also faced cuts.

Until now, Prime Minister David Cameron's government has said replacing the aging fleet of four submarines that carry nuclear warheads to provide a continuous at-sea deterrent would cost an estimated 15-20 billion pounds.

It has as yet given no official estimate of the cost of its replacement and maintenance.

Critics have previously said Britain would need to spend 100 billion pounds, a figure based on a 2014 report by the independent Trident Commission.

In a written parliamentary response to Crispin Blunt, a lawmaker in Cameron's Conservative Party, Minister of State for Defense Procurement Philip Dunne said Friday the acquisition of four new submarines would cost 25 billion pounds.

He added that the in-service costs would be about 6 percent of the annual defense budget over their lifetime. The total defense budget for 2014-15 reached 33.8 billion pounds and rises to 34.1 billion pounds for 2015-16, according to the ministry.

"My office's calculation is based on an in-service date of 2028 and a missile extension until 2060 ... the total cost is 167 billion pounds," Blunt told Reuters.

"The successor Trident program is going to consume more than double the proportion of the defense budget of its predecessor. ... The price required, both from the U.K. taxpayer and our conventional forces, is now too high to be rational or sensible."

His figure is based on the presumption that Britain will spend 2 percent of its annual gross domestic product (GDP) on defense as Cameron's government has promised.

It also uses existing official government and International Monetary Fund figures, and an assumption of GDP growth of an annual average of 2.48 percent between 2020 and 2060.

Using the same figures, a Reuters calculation came to the same sum of 167 billion pounds.

Asked about the rising cost, a spokesperson for the British Ministry of Defense said the government had published an unclassified version of a review on alternatives to Trident that "demonstrated that no alternative system is as capable, or as cost-effective, as a Trident-based deterrent".

"At around 6 percent of the annual defense budget, the in-service costs of the UK's national deterrent ... are affordable and represent an investment in a capability which plays an important role in ensuring the UK's national security," the spokesperson said.

Dangerous Obsession

The deputy leader of the SNP, Stewart Hosie, took aim at the Conservatives, or "Tories," saying the new figure showed "just how dangerous the Tories' obsession with nuclear really is".

"This is truly an unthinkable and indefensible sum of money to spend on the renewal of an unwanted and unusable nuclear weapons system," he said in a statement.

The SNP's popularity has surged since Scots rejected independence in a vote last year, with millions of supporters won over by its anti-austerity message and criticism of Trident.

The opposition Labour Party had been a supporter of renewal, but its new leader, far-left veteran lawmaker Jeremy Corbyn, an anti-war campaigner, is opposed to the plans.

He was widely quoted last month as saying he would not be prepared to use nuclear weapons if he became prime minister.

Spiraling costs are likely to reinforce Corbyn's opposition and possibly alarm many in his party who support renewal.

The new figures tally with comments this month by Jon Thompson, the top civil servant at the Ministry of Defense, when he described the project to replace the nuclear deterrent as a "monster."

"That's the project that keeps me awake at night the most," he told parliament's Public Accounts Committee.

"It's the biggest project the Ministry of Defense is ever going to take on. If the government were to proceed with renewing the deterrent, then in due course that would exceed 5 billion [pounds] a year. That is a significant proportion of the defense budget, and it's an incredibly complicated area."

He added that it is extremely difficult to estimate what the future costs will be.

A final decision on replacing the existing vessels carrying the Trident missiles -- four Vanguard-class submarines -- is due next year, and Cameron says he will press ahead with the renewal.

In August, the government said it would spend more than 500 million pounds refurbishing its Faslane naval base in Scotland.

"I think it is right to maintain our independent nuclear deterrent, and anyone who has any doubts of it only has to look at the dangers and uncertainty in our world," Cameron told parliament Wednesday.

In a speech last week, Defense Secretary Michael Fallon said global threats mean renewing Trident is vital.

"I appeal to all moderate MPs [lawmakers] to put our national security first and to support building four new Trident submarines," he said. "Spread across the 30-year life of the new boats, this represents an annual insurance premium of around 0.13 percent of total government spending."