ALEX Salmond plans to draft a new constitution for an independent Scotland which explicitly bans nuclear weapons from the country.

The move is an attempt to reassure those in the SNP who doubt that Trident can be removed from the Clyde for good.

If Scots vote Yes in 2014, the nation's founding constitutional document will include a clause declaring weapons of mass destruction illegal on Scottish soil and in Scottish territorial waters.

The ban would give the Scottish Government extra leverage in negotiations with London and the US on Trident, as refusal to remove the submarines and warheads from Faslane and Coulport would be regarded as a breach of sovereignty and provoke international condemnation.

Such a move has been tried before by other nations. As part of its constitution, the Philippines has a "policy of freedom from nuclear weapons in its territory", while Austria, Mongolia and New Zealand have passed laws banning their use or storage.

At their conference yesterday, the Scottish Greens also voted for a ban on nuclear weapons to be part of the constitution.

Although in line with longstanding SNP opposition to nuclear weapons, Salmond's proposal is timed to avoid an embarrassing defeat at the party's conference on October 21. Salmond and defence spokesman Angus Robertson face growing opposition to their plans to end the party's 30-year-old policy on leaving Nato.

To counter Unionist claims an independent Scotland would be left poorly defended, Robertson is pushing for Nato membership subject to an agreement to expel Trident in future. His pro-Nato motion has the support of Salmond and other Cabinet secretaries.

But many SNP members, including a dozen MSPs, are fiercely opposed to staying in Nato, as they see it as synonymous with nuclear weapons.

The mutual defence agreement between Nato's 28 member states extends to the use of nuclear arsenals held by the US, the UK and France. Sceptics fear if Scotland joined Nato the alliance would stall for years over whether or not to remove Trident – something which happened to other Nato members wanting rid of US weapons on their soil.

Given the lack of an alternative site for Trident outside Scotland, its expulsion would effectively disarm the rest of the UK, a development Nato might well resist.

To defeat Robertson's conference motion, Cumbernauld MSP Jamie Hepburn has tabled a rival amendment, which if adopted would see the SNP keep its policy of quitting Nato.

Robertson and Salmond had expected to win the vote with ease. But they have been taken aback by a groundswell of opposition among SNP branches, many of which recently mandated their conference delegates to oppose any change in the Nato policy.

The idea of a constitutional ban, developed by Robertson, is meant to reassure activists Trident would be expelled even if Scotland stayed in Nato, and help Robertson and Salmond win the vote at conference.

A senior party source said: "There would be a constitutional provision that weapons of mass destruction, nuclear weapons, would be illegal under the terms of the constitution.

"It underlines the absolute commitment of the SNP to removing nuclear weapons. Angus wants to underline the point that Nato membership would be conditional on Scotland not having nuclear weapons."

However, the face-saving exercise may not work. Sceptics last night insisted that, while a constitutional ban was a good plan, it did not end the arguments over Nato, its weapons and its operations.

Hepburn told the Sunday Herald: "I think it's a great idea, but it's not entirely relevant to the debate about whether or not we remain in Nato after independence.

"Are we against nuclear weapons just because they happen to be in Scottish territory, or are we against them in principle? Are they a mistake here but okay in Germany or Holland? Nato would continue to possess nuclear weapons through its other member states."

Skye SNP MSP Dave Thompson added: "I would expect [a no nuclear weapons clause] to happen in any case. It does not address the Nato issue."

The SNP last proposed a constitution for an independent Scotland in 2002. It did not mention nuclear weapons.

John Ainslie, Scottish CND co-ordinator, welcomed the proposal for a constitutional ban on nuclear weapons. "It helps makes the process for removing Trident as substantial as possible. Governments come and go and things change, and although it's always possible to change constitutions, it's far harder. It means [Scotland's non-nuclear status] is established as a firm principle."

Jane Tallents, of disarmament group Trident Ploughshares, said: "We would be delighted for this to be written into the constitution. But it's dispiriting that the SNP also wants to be in an alliance where nuclear weapons are fundamental. Morality doesn't stop at the border."

Leonna O'Neill, of the No To Nato Scotland Coalition, said a constitutional ban would be welcome, but would be hypocritical if an independent Scotland signed up to Nato.

She said: "I would support anything the SNP produces that sets a ban in a legal framework to get rid of Trident, but it's unacceptable to join Nato regardless. It's like saying you won't carry a knife, but you still hang about with a gang of other people carrying knives."