TENSIONS inside the SNP about whether to end its historic opposition to Nato spilled over yesterday as MSPs rounded on the party's defence spokesman for failing to justify the proposed shift, and suggested he was trying to change policy based on opinion polls rather than principle.

Angus Robertson was accused of seeking "electoral advantage" in the independence referendum instead of sticking to a policy that was right morally.

Robertson, who is backed by First Minister Alex Salmond, is trying to persuade the party to drop its 30-year-old opposition to Nato membership in a vote at the annual conference in October.

The change is designed to ease voter concerns, and neutralise Unionist criticism, that an independent Scotland would be left unprotected.

However, Robertson's proposal is fiercely opposed by many SNP members, who deplore Nato's reliance on nuclear weapons, and believe joining would make it virtually impossible for an independent Scotland to remove Trident from the Clyde.

Ten of the SNP's 67 MSPs have so far come out against Robertson's plan, and yesterday five of them gave their first joint press conference at a meeting of the SNP's CND group in Glasgow. To undermine the event, Robertson, MP for Moray and leader of the SNP at Westminster, issued poll findings overnight that suggested 63% of voters felt Scotland would be safer in Nato.

The MSPs were scathing about the poll and Robertson's failure to explain his big idea.

Asked why he thought Robertson was trying to change policy now, Dave Thompson, MSP for Skye, Lochaber and Badenoch, said: "He obviously feels that there would be some electoral advantage. I don't agree with him. I think we're better with our current policy because it's the right policy.

"You would need to ask Angus whether he thinks his policy is the right policy, or is he purely following opinion polls. That's not for me to say."

Thompson went on: "Whatever Angus's reasons are, he needs to let us know what they are. I don't think he can just use the argument that an opinion poll says 63% of the public favour Nato, therefore we have to go that way. He's got to come up with sound reasons."

Emphasising that she was guided by principle, Glasgow Kelvin MSP Sandra White added: "Angus is worried, otherwise he wouldn't have been so [vocal] in the newspapers, coming out with this poll.

"I would dearly like to ask Angus Robertson what either he, or the SNP, or the Yes campaign for independence can possibly gain by being a member of Nato, and I'll await that answer."

Highlands and Islands MSP John Finnie said: "It's for the proponents of change to say how they believe that the ultimate goal, the removal of nuclear weapons, will be enhanced by this ... Maybe Angus is party to information. Maybe Angus has had high-level discussions with Nato representatives. If so, maybe it would be helpful for that to be shared."

Highlands and Islands MSP Jean Urquhart said she and her colleagues wanted to stick with the SNP policy of withdrawal from Nato "because we think it's right, not because we think it's populist or popular".

Central Scotland MSP John Wilson said: "Why Angus decided to bring this debate forward at this time ... you need to get Angus to answer those questions.

"Certainly none of us could answer questions about this issue being raised now."

In his motion to conference, Robertson will ask the party to agree that: "An SNP government will maintain Nato membership subject to an agreement that Scotland will not host nuclear weapons and Nato continues to respect the right of members to only take part in UN-sanctioned operations."

However, sceptics believe that once Scotland was in Nato, the US and UK would stymie removal of Trident with endless bureaucratic delays. They cite the example of Nato members Germany, Belgium and the Netherlands, who have all asked in vain for US weapons to be removed from their soil.

There are also concerns that even if Scotland was no longer to "host" nuclear weapons, Nato membership could result in the armed submarines of other Nato members using her waters.

In a possible sign of things to come, earlier this month Le Triomphant became the first French nuclear-armed submarine ever to dock at Faslane.

Nato was founded in 1949 on the principle of collective defence, and its 28 members vow to defend each other from external attack, up to and including the use of nuclear weapons by France, the UK and USA.

Nato itself says that, "As long as nuclear weapons exist, Nato will remain a nuclear alliance".

In response to Robertson's motion, Cumbernauld and Kilsyth MSP Jamie Hepburn recently tabled a rival amendment seeking to keep current policy, which is for an independent Scotland to quit Nato.

Some SNP activists see the argument over Nato as being about the broader culture of the SNP. They fear Salmond and the leadership want to dump any controversial policies ahead of the referendum and may yet soften the line against nuclear weapons.

At yesterday's SNP-CND group, about 50 activists discussed how to build resistance to the Robertson plan through the party's branch network, and debated a paper claiming Nato membership would cost the SNP its vision of Scotland as "young, vibrant and peaceful" and instead make the party look "cynical and lacking in moral courage".

Dave Thompson said the lack of any suitable base for Trident outside Faslane meant that an independent Scotland could be the first domino in a new phase of global disarmament.

"If we get rid of them in Scotland, we get rid of them in the UK. That leaves Nato with only two countries with [their own] nuclear weapons, that's the USA and France.

"This could be the start of nuclear disarmament within Europe, because if they see a country like the UK not having nuclear weapons, a country that was right in there at the start more or less, that would add pressure on the States to remove their weapons from Germany and Holland and Belgium.

"That could in turn lead to a real strong move to nuclear disarmament across the world – maybe force people to take the Non-Proliferation Treaty seriously, as we move into the 21st century."

Scottish Conservative leader Ruth Davidson said: "It is becoming increasingly clear that this proposed U-turn on Nato is a political one made in the interests of conning voters ahead of 2014.

"It isn't being proposed in the interests of the safety of Scottish people, or the security of our brave servicemen and women. The ridiculed Nato stance is only one small element of SNP defence policy that is exceptionally weak."

An SNP spokesman said: "The SNP has a cast-iron commitment to rid Scotland of nuclear weapons. Given the continued presence of Trident nuclear weapons in Scotland – against the wishes of the Parliament and people – independence is the only constitutional option which makes this possible.

"SNP members have the democratic opportunity to make their views on Nato membership clear at the annual Conference in October, where we are going to have an excellent debate.

"The factual position is that Nato doesn't decide where nuclear weapons are based – weapons from one country that are stationed in another are only there by bilateral, mutual agreement."

The case for leaving Nato: Dave Thompson

The case for staying in Nato: Angus Robertson