Gordon Brown today offered to put Britain's Trident nuclear missiles on the international negotiating table to signal that he is serious about playing a leading role in multilateral disarmament talks.

In a speech this morning at Lancaster House, the prime minister said that "as soon as it becomes useful for our arsenal to be included in a broader negotiation, Britain stands ready to participate and to act".

Nuclear arms experts said that the speech signalled a significant change in Britain's stance, showing a greater readiness to put Trident on the negotiating table. Since a landmark speech in June 2007 by the then-foreign secretary, Margaret Beckett, Britain has officially endorsed the ultimate aim of abolishing nuclear weapons, but has been accused of hypocrisy because of its decision to renew and modernise its submarine-based Trident deterrent.

"The British government has to get over its lack of credibility on the international scene. This is their attempt to establish their credibility," said Paul Ingram, the executive director of the British American Security Information Council. He said that the previous British position had been that Trident would not be put on the table until US and Russian arsenals had been reduced to roughly the same levels.

"Brown is saying that Britain is now prepared to put Trident into the mix at an earlier stage," Ingram said.

The Obama administration has signalled its willingness to make deep cuts in the US arsenal, and – Dmitry Medvedev's speech today to Russia's generals notwithstanding – Moscow has suggested it may be willing to respond. Some reports suggest that the world's two major arsenals could be reduced to under a thousand warheads each. Britain has 160 warheads deployed and ready for use at any one time, but Brown said that number was "under constant review".

"If it is possible to reduce the number of UK warheads further, consistent with our national deterrence requirements and with the progress of multilateral discussions, Britain will be ready to do so," the prime minister said.

A Downing Street official said it would be unhelpful for Britain's negotiating position to say at what point Trident would be put on the table in disarmament talks, but he said the speech was intended to show "we are serious and not just paying lip service".

"If we are serious and we do actually want to energise the discussion [on multilateral disarmament], we need to take an open position on our own nuclear deterrent," the official said.

The 1968 non-proliferation treaty (NPT) is up for review next year at a time when it is under increasing strain. North Korea tested a nuclear device in 2006, and Iran is insisting on its right to enrich uranium in the face of UN sanctions, denying western claims that it is developing weapons. Israel already has a nuclear arsenal, and other Middle Eastern countries are looking into acquiring one.

Non-nuclear states such as Iran have argued that the world's declared nuclear powers have not fulfilled their part of the NPT, failing to carry out significant disarmament.

The prime minister yesterday said Britain would lead a programme of intensified negotiations leading up to the NPT review conference, under the title "Road to 2010".

"As possessor states we cannot expect to successfully exercise moral and political leadership in preventing the proliferation of nuclear weapons if we ourselves do not demonstrate leadership on the question of disarmament of our weapons," he said. "We are not asking non-nuclear weapons states to refrain from proliferation while nuclear weapons states amass new weapons. We are asking them not to proliferate while nuclear weapon states take steps to reduce their own arsenals in line with the non-proliferation treaty's requirements."

He restated the west's offer to help Iran to build a civil nuclear industry under full UN safeguards, including establishing a guaranteed supply of nuclear fuel, if Tehran suspends uranium enrichment. If the Iranian government continued to refuse, Brown said, it would face tougher sanctions.