Nick Clegg has become the most senior political figure to declare that he is opposed to the renewal of Trident, Britain's nuclear weapons system.

The Liberal Democrat leader argued that in the post-cold war world there was no need for such a large deterrent and that growing public debt meant that a £20bn renewal was unreasonable.

The policy reversal means the main parties are divided over Britain's nuclear weapons for the first time in decades. Labour and the Conservatives remain committed to Trident renewal.

In an interview with the Guardian, Clegg said: “New leadership in Russia, new leadership obviously in the White House and a wider geostrategic appreciation means that a Cold War missile system designed to penetrate Soviet defences and land in Moscow and St Petersburg at any time, in any weather, from any location anywhere round the planet, is not our foremost security challenge now”.

He added: “We have got to be grown-up and honest about it. Given that we need to ask ourselves big questions about what our priorities are, we have arrived at the view that a like-for-like Trident replacement is not the right thing to do”.

However, Clegg suggested that Britain could arm the Navy's Astute class submarines with nuclear missiles or, like Japan, become 'virtually nuclear', allowing nuclear material to be weaponised within six months.

He revealed that he had asked Sir Menzies Campbell, the former party leader, to hold a review into how Britain could maintain a reduced deterrent but added that it would be an “unhappy event” if full disarmament was not achieved.

He hopes to agree to coordinated disarmament with other countries in multilateral negotiations next year.

Clegg had previously argued for renewal, setting himself against leadership rival Chris Huhne who opposed Trident. But he conceded that “the facts have changed, you've got to change with them”.

He rejected claims that disarmament would threaten Britain's permanent seat on the UN security council as “nostalgic, sepia-tinged phooey”.