LONDON — Britain will need to find an extra £6 billion (US $9 billion) a year by the end of the decade to keep its defense budget in line with the NATO mandated spending level of 2 percent of gross domestic product (GDP), a former defense minister said.

Nick Harvey, the Liberal Democrat defense spokesman in Parliament and the Armed Forces minister between 2010 and 2012, said figures that emerged last week in a think tank report on potential British cuts were pretty much in line with his understanding at the time of his departure from the MoD.

"The Royal United Services Institute [RUSI] work accords very much with my recollection of the figures when I left the MoD over two years ago. ... By the end of the [next] Parliament, with no more defense cuts at all, the gap between the defense budget and what is likely to be 2 percent of the GDP by 2019 will be getting toward £6 billion," Harvey said.

"This is why neither George Osborne [the chancellor of the Exchequer] or anybody else has been in any great hurry to commit firmly to the 2 percent because of the sums of money which will have to be found to achieve that," he told reporters during a briefing March 11.

None of the leading political parties have so far committed to maintaining NATO spending levels beyond the next financial year.

Ahead of a general election scheduled for May 7, a major row has broken out here over potentially significant defense spending cuts as part of an expected new round of austerity measures.

"We know a lot more needs to be done to tackle the deficit and as yet nobody [in government] has come forward and suggested defense can be protected from that, and therefore some very painful decisions look likely to be facing whoever the ministers are," Harvey said. "If you think we faced some pretty unpalatable choices in the 2010 strategic defense and security review, and we certainly did, then I can see it is going to be every bit as difficult this time around."

In 2010, the government axed Harrier jump jets, maritime patrol aircraft, surface warships and other capabilities as well as slashing equipment programs and personnel numbers as part of an effort to reduce defense spending.

Harvey added, however, said though that like in 2010, defense would not "get the worst of what's coming in the comprehensive spending review because it's an unstable world."

A government spokesperson declined to address the possible £6 billion shortfall but said, "Decisions on spending after the financial year 2015-16 will be determined in the next spending review."

The question of whether Britain will continue to meet or beat the NATO spending target has become a central issue in the row that has involved politicians, retired senior officers and others. Even US President Barrack Obama and US Army Chief Gen. Ray Odierno have voiced their concerns.

British Defence Secretary Michael Fallon visited Washington last week for discussions with his new US counterpart, Ashton Carter, and used a speech at a think tank to try to and counter the growing view that UK defense capabilities are diminishing to the point where, as Odierno said, British forces would have to fight inside American formations rather than alongside.

The Liberal Democrats are the junior partner in the Conservative-led government and could play a role in the formation of the next administration. The opinion polls here are showing a coalition or a minority government could emerge after May 7.

RUSI said that in the "context of the wider austerity in public spending, such an increase [which the think tank estimated marginally lower at £5.9 billion, rather than Harvey's £6 billion figure] is not plausible."

Rather than an increase, RUSI said it was likely there would be cuts and mapped out two scenarios citing possible spending reductions of between 8.7 percent and 17.5 percent against a baseline figure for 2016-17 of £34.3 billion.

In a March 12 parliamentary debate on defense spending levels, Conservative lawmaker John Baron said that while the NATO figure was arbitrary and spending should reflect the capabilities desired, "the 2two percent does have a symbolic value. Having lectured other NATO members [at the NATO summit last September] about its importance, we should lead by example."

The British are hovering on the cusp of the 2 percent mark for this year and are expected to fall below that the mandated level for the first time next year.

The US, Greece, Estonia and Britain are the only NATO nations that currently meet the required spending level.

Last week it emerged the Conservatives were trying to finesse spending numbers by including items such as , like military pensions, which had have not previously been included in the defense budget.