ALEX SALMOND has made a major bid to win international backing for his government's campaign to rid Scotland of nuclear weapons, the Sunday Herald can reveal.

The first minister has written to 122 countries highlighting the nation's opposition to the deployment of Trident nuclear warheads on the Clyde, and his determination to try and block the UK government's decision to replace Trident.

Salmond is also asking the countries to support a request for Scotland to be given observer status at future meetings of the parties to the Nuclear Non-Proliferation Treaty (NPT), an international agreement to limit the spread of nuclear weapons.

The first minister's move, on the eve of the Scottish government's Trident summit in Glasgow tomorrow, has been hailed as a potential breakthrough by disarmament experts.

Tomorrow's meeting will hear evidence from a Scottish advocate, John Mayer, that nuclear weapons are illegal under international law and the Scottish parliament would be "well within its competence" to pass legislation preventing crimes committed by weapons of mass destruction.

"The law of the whole world is against Trident. The Scottish parliament stands at a turning point to outlaw its threat or use, and in doing so will lead the world into a safer 21st century," said Mayer.

Salmond's letter, sent last week, is addressed to the UK ambassadors of 122 countries party to the NPT. "In May, for the first time since the nuclear age began in 1945, the people of Scotland elected a government that is opposed to nuclear weapons," he declared.

The Scottish government was planning to do "all that we can" to persuade UK ministers to change their mind on Trident, said Salmond. "The majority of Scottish people and their elected representatives oppose these deployments."

A decision to replace Trident was taken by former prime minister, Tony Blair, and pushed through the Commons in March, despite a Labour revolt. But a majority of Scottish MPs opposed the decision, as have an overwhelming majority of MSPs.

Tomorrow's summit will be opened by the deputy first minister, Nicola Sturgeon, and closed by the Scottish minister for parliamentary business, Bruce Crawford. "There are few more important issues in the world today than nuclear weapons proliferation," Crawford told the Sunday Herald.

"As a country we have every right to give voice to our opposition to nuclear weapons on Scottish soil."

Scotland's stance was welcomed by disarmament experts. "This bold and timely initiative deserves to succeed," said Dr Ian Davis, director of the British American Security Information Council in London. He argued that there was a growing global movement to reverse reliance on nuclear weapons, backed by former US secretaries of state, George Schulz and Henry Kissinger, as well as Democratic presidential candidates Barack Obama and Hillary Clinton.

He said: "The Scottish people have long held the moral high ground on nuclear disarmament and having them represented at the NPT would be a real fillip to the majority world - currently 184 states - committed to a non-nuclear weapon future."

Dan Plesch, director of the Centre for International Studies and Diplomacy at the University of London, said Britain was violating the NPT by acquiring nuclear weapons from the US and by failing to conclude a treaty banning them, "so this Scottish initiative is bound to be welcomed internationally".

Green MSP Patrick Harvie, who will speak at the summit, said: "We all have an obligation to do what we can to stop the Westminster government forcing another generation of nuclear weapons onto Scotland."

New research released by the Scottish government suggests that opposition to nuclear weapons was a significant factor in causing voters to switch from Labour to the SNP at the May election.

There was "a compelling case Trident was at least part of the reason for Labour losing support in 2007", said the author of the research, Robert Johns from the University of Strathclyde.

A YouGov poll in May showed 58% of people in Scotland opposed Trident, up four to six points on previous surveys.