Pressure is mounting on Theresa May to sign up to a UN treaty calling for the eventual elimination of nuclear weapons.
More than 120 nations endorsed the global treaty at a summit in July, which warns that a complete ban is the only way to prevent the “catastrophic humanitarian consequences” of the use of nuclear weapons.
Britain and other nuclear nations opposed the move, but critics have called on the Prime Minister to change course this week when the treaty will be opened for signatures at the UN’s annual general assembly.
It comes amid escalating tensions between the US and North Korea, after a string of nuclear tests from the pariah state and war-like rhetoric from Donald Trump.
Anti-nuclear campaigners called on Britain to take the lead on disarmament, or risk offering a “blank cheque” to other nations seeking to boost their nuclear arsenal.
The UK and other nuclear powers support a non-proliferaton treaty, which prevents the spread of nuclear weapons – but the pact has previously attracted criticism for being ineffective.
Green Party co-leader Caroline Lucas said the Government had “utterly failed” in its commitment to nuclear disarmament, and urged the Prime Minister to back the UN treaty, in which signatories agree not to develop, test, buy or possess nuclear weapons.
She said: “Britain’s refusal to engage with the nuclear ban treaty is shameful. While the vast majority of the world’s countries are signing it our Government is refusing.
“The Government says it’s committed to multilateral disarmament and yet when presented with the opportunity to demonstrate it, they’ve utterly failed.
“Imagine what it would mean for Britain’s standing in the world if we became the first nuclear weapons state to sign this treaty, and commit to ridding this country of these monstrous weapons of mass destruction. We have a real chance to be leading the world at this most pivotal moment.
“The Government must commit to this Treaty on the Prohibition of Nuclear Weapons, and do it now. To ban these weapons would offer the best possible protection from them for future generations.”
Labour also urged the Prime Minister to send a strong message over the UK’s nuclear ambitions.
Shadow Defence Minister Fabian Hamilton said: “Labour is committed to working towards a world free from nuclear weapons, and we take the diplomatic efforts towards achieving this seriously.
“The growing tensions on the Korean Peninsula reminds us why we need nuclear armed states to re-engage with the process of meaningful multilateral disarmament.
“In the United States next week, the Prime Minister has the opportunity to constructively engage in the United Nations talks.
“I urge her to send a strong message about our country’s commitment to uphold our international obligations under the Nuclear Non-Proliferation Treaty.”
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Andrew Smith, of the Campaign Against Arms Trade, said: “For far too long, UK foreign policy has been guided by an unbending commitment to militarism and interventionism. Trident has been right at the heart of it.
“Despite this, there is a growing international consensus against nuclear weapons.
“It’s time for Trident-owning countries like the UK to take a lead, and take a crucial step towards a nuclear-free world.
“To continue doing otherwise will only provide an excuse and a blank cheque for every other country that seeks nuclear proliferation.”
Downing Street was unavailable for comment.