Britain committed Tuesday to retaining its nuclear deterrent but said it would cut the number of warheads it has and delay a politically-sensitive decision on building new submarines until 2016.

Unveiling the country's first defence review in 12 years, Conservative Prime Minister David Cameron said the Trident nuclear programme was "the ultimate insurance policy" and was key to maintaining national security.

But amid opposition from the Liberal Democrats, the junior partner in the coalition government which took power in May, Cameron said no decision would be taken on a replacement until after the next election in 2015.

In what could be seen as a further concession to the Lib Dems, the defence review also pledges to reduce the number of nuclear warheads carried by the existing nuclear submarines from 48 to 40 per vessel.

This will reduce Britain's requirement for operationally available warheads from 160 to 120 and cut its overall nuclear weapon stockpile from 225 to no more than 180 by the mid 2020s.

"We will retain and renew the ultimate insurance policy -- our independent nuclear deterrent, which guards this country round the clock, every day of the year," Cameron told the House of Commons.

He added: "But as a result of the changes to the programme, the decision to start construction of new submarines need not be now taken until around 2016."

Defence chiefs estimate that the existing Vanguard class submarines can be safely operated until the late 2020s, which means any replacement need not be introduced until 2028.

As a result, the government can defer a decision on the new subs until after the five-year period covered by the defence review, saving both the costs of the replacement programme and avoiding a potential fall-out with the Lib Dems.

The agreement which underpins cooperation between Cameron's Conservatives and the Lib Dems only lasts until the next election which must be held by 2015.

The defence review set out Britain's security strategy and also outlined cuts of eight percent to the Ministry of Defence's budget, as part of sweeping savings across in government to pay off a record budget deficit.