First Minister Alex Salmond's military spending plan for Scotland "completely undermines" the opportunity to take a different approach, according to critics in the pro-independence campaign.

Green and independent MSPs want the SNP leadership to rethink a proposed £2.5 billion annual defence and security budget as part of the Nato nuclear defence alliance.

The SNP changed its policy last year to support membership of Nato, assuming a Yes vote in the referendum in September next year.

The decision led to two MSPs, Jean Urquhart and John Finnie, leaving the SNP and joining a new Green/independent group at Holyrood, which also includes former SNP politician Margo MacDonald.

In a jointly-signed letter to Mr Salmond, they wrote: "The UK commits £39 billion of public money each year to military expenditure whilst vital public services face drastic spending cuts. It is our firm belief that this is the wrong choice.

"Our public resources should be focused on challenging society's inequalities, improving people's health and well-being, and helping create an environmentally-sustainable society, and we hope you share that view.

"Independence will give Scotland a chance to do things differently but the SNP's plan to commit £2.5 billion to an annual defence and security budget as part of Nato completely undermines this opportunity.

"Clearly there is a role for national defence but this does not require membership of a cold war nuclear alliance and a budget running into billions of pounds.

"The last thing an independent Scotland should do is mimic the military posture of the UK, with the aggressive projection of power around the world and engaging in reckless overseas adventures."

They accept that a smaller budget would affect communities that rely on the defence industry but argue money should be targeted to help diversify their local economies in areas such as energy technology.

The group backs the SNP's position that Trident should be removed from Scottish waters.

The call comes before a Scrap Trident march in Glasgow tomorrow and a planned blockade of Faslane, the home of Britain's nuclear deterrent, on Monday.

Green leader Patrick Harvie said: "This weekend of action shows the appetite to rid Scotland of this horrific nuclear weapons system has grown stronger and stronger.

"A debate led by the Scottish Greens in the last session of parliament resulted in a historic vote against the renewal of Trident and we continue to call for communities reliant on military jobs to be helped to diversify.

"It is vital that those of us who want to see Trident scrapped also make the case against outdated overblown military budgets and pro-nuclear clubs like Nato.

"An independent Scotland that took a different tack from the UK would send a very powerful message to the international community about the folly of military aggression."

Mr Salmond argued the case for Scotland remaining in Nato during a visit to the US this week.

But his ambition for Scotland to become a member of the alliance in its own right before formally leaving the UK was questioned by his political opponents. They responded to a statement from Nato that Scotland would have to reapply after independence.

An SNP spokesman said: "In an independent Scotland, all political parties can put forward their defence and other policies and the people will choose - that is the beauty of independence and the key principle that unites the Yes campaign.

"Crucially, independence is the only constitutional option that enables us to get rid of Trident, as over thee-quarters of people in Scotland want, instead of it being dumped on the Clyde for another 50 years by Westminster.

"Under Westminster control, Scotland contributes £3.3 billion to UK defence spending right now but receives only some £2 billion in return.

"An independent Scotland with a defence budget of £2.5 billion - half a billion more than Scotland gets from Westminster, but nearly a billion less than we currently pay - means more jobs and economic activity in Scotland.

"That is the SNP's policy, democratically passed by the party conference last October after a full and thoughtful debate."