THE SNP is mishandling the independence debate and needs to change its approach to win in 2014, a respected arm of the Yes campaign warns today.

With support for a Yes vote sliding in the polls, and the Unionist parties escalating their attacks, the Scottish Independence Convention (SIC) has published a previously secret paper on the state of the campaign, which even says SNP attitudes could be a "barrier" to success.

The report says the independence movement can appear weak, divided and nervous, and warns that a potential backlash against the SNP hogging power risks creating "a fatally fragmented campaign".

Instead of trying to answer every question posed by the unionist parties, it recommends a new focus on the principles which will be used to resolve the future detail of independence.

Founded in 2005 as a cross-party vehicle for separation, the SIC includes members of the SNP, Greens, Scottish Socialists and Solidarity, and is chaired by actress Elaine C Smith.

One SIC source said the paper, which has been circulated among the SNP hierarchy for several weeks, was being made public to snap the SNP out of a "death spiral", with the last straw being the recent row over Scotland's status in the EU.

The source said: "The reason we're putting this out is that we're losing and everybody knows it. Everybody in the indy movement knows we're going backwards, not forwards. The SNP is panicking."

The SIC paper has been given a guarded welcome by Dennis Canavan, chairman of Yes Scotland, the main campaign group for independence, which until now has largely mirrored SNP thinking.

Better Together, the pro-Union campaign for a No vote, praised the paper's "honesty".

The paper says the SNP-dominated Yes campaign has fallen into a trap laid by Unionists, who have turned the debate into an endless series of questions which can never be answered in full. By engaging with this, the Yes camp's answers have been "nervous, cautious and conservative", angering campaigners who want radical change. The situation is affecting campaign strategy and policy development "in a manner likely to weaken the case for independence", it concludes.

The SIC urges shifting the debate on to the broad principles by which issues would be resolved, rather than trying to supply every last detail, saying the focus should be on "ideas, debate and possibility, not facts, data and inevitability".

It also says the Scottish Government White Paper on the mechanics of independence, due in November 2013, should be produced "consensually" by the whole independence movement, and should avoid specific SNP policies.

This could prove contentious, as other members of the Yes campaign, such as the Scottish Socialists and Greens, fundamentally disagree with the SNP over keeping the pound and the monarchy.

The SIC also identifies three SNP traits as potential barriers to success: a reluctance to share control of the Yes campaign; an instinct to hedge its bets in case of a No vote; and a desire to promote its own policies as if they were givens in an independent Scotland.

It says: "The SNP must either decide to fight the referendum to win or fight it with half an eye of winning a Scottish election if the referendum is lost.

"The SNP exists to achieve independence; it must cede some control of the process if it is to achieve its purpose. The alternative is a fatally fragmented campaign."

Canavan said: "I personally do not endorse the entire contents of the paper, but I think it is an important contribution to the debate about Scotland's future. It draws a very relevant distinction between those issues which are best settled by negotiation, those which are best included in a Scottish Constitution and those which are best decided at the first elections to an independent Scottish Parliament."

Better Together said: "The SNP may want the people of Scotland to vote for a pig in a poke; we won't allow them to get away with it."

Asked about the criticisms, the SNP said: "The Scottish Government will publish a White Paper in November 2013 setting out ministers' detailed proposals for independence."