Glasgow Shettleston MSP John Mason, who initially said he would await the views of his constituency party before declaring, became the latest Nationalist to join the revolt.
Yesterday Mr Mason said he was now "95% likely" to vote against the party hierarchy on the issue at the annual conference in October. "I think there is a serious concern about this issue," said Mr Mason.
"Everyone realises this is a major decision, and I think there will be a spectacular opportunity at both ends, from people who are strongly for or against the motion, to those who simply feel it is an important decision for the party to make."
Mr Mason's intervention follows The Herald's report yesterday that a paper presented at the party's CND group conference in Glasgow on Saturday will claim the policy change will cost the SNP its vision of Scotland as a "young, vibrant and peaceful proposition". It is likely to "put off those who support a peaceful prospectus (such as women and the young) but is unlikely to do much for people who like guns and bombs" while making the SNP "look cynical and lacking in moral courage".
The report concludes: "It alienates almost all active supporters of independence outside the SNP and will cause more conflict within the Yes campaign. It removes all differentiation between the SNP and the other parties."
Jamie Hepburn, the MSP for Cumbernauld and Kilsyth, has moved October's anti-Nato amendment to the motion from Angus Robertson, the party's defence spokesman.
MSPs John Finnie, Sandra White, Jean Urquhart, Dave Thompson, Bob Doris, Marco Biagi, John Wilson and now Mr Mason oppose the move by Mr Robertson, who believes a pledge of continuing membership of Nato will be another reassuring pitch in appealing to Scots voters in the 2014 referendum, arguing that Norway is both a non-nuclear state and alliance member.
But critics point out Norway has never been a nuclear state, putting it in a very different position from that which Scotland would find itself in with nuclear weapons already on its territory.
The report states:
l If Scotland wants to stay in Nato then it will be more difficult to get rid of Trident.
l Three European countries want US nuclear weapons removed from their soil, but this has been obstructed by Nato.
l The alliance would delay, possibly indefinitely, if Scotland asked to stay in Nato on the condition that Trident must go.
l If Trident is removed from Scotland there will be no nuclear weapons in Britain, something Nato is likely to resist.
The document says: "The proposal to change SNP policy may be well-intentioned, but it fails to take account of the reality of Nato politics.
"If the government of an independent Scotland went to Nato with the proposal that they wanted to remain in the alliance without Trident, they would not, as former Nato Secretary General George Robertson has suggested, be told No. But neither would they be told Yes.
"The question would be lost in the byzantine decision-making process of the alliance. Several members of Nato, including the Rest of the UK and the US, would be keen to postpone any answer.
"Their goal would be to retain Scotland within the alliance, with Trident still at Faslane. They would explain Scotland's long wait for an answer as the inevitable result of the need to retain cohesion within the alliance."