SNP ministers have been accused of trying to rig the independence referendum by imposing tough spending limits on the pro-UK parties.

In a speech to the SNP conference in Perth, Deputy First Minister Nicola Sturgeon gave the clearest indication yet that the Scottish Government is preparing to disregard impartial advice on campaign funding from the Electoral Commission.

The neutral watchdog – handed a key role in setting referendum rules under the terms of the Edinburgh Agreement last week – favours higher spending limits for the two main campaign groups than those proposed by ministers.

The commission also wants higher spending caps for political parties under a formula which the SNP claims would put it at a £1 million disadvantage.

Closing the Nationalists' conference yesterday, Ms Sturgeon said: "We will set the spending limits with care. We will listen to our consultation. We will listen to the Electoral Commission. We will listen to both campaign organisations.

"And then we will take a decision that is right for Scotland, a decision that guarantees a level playing field and a fair contest."

To applause, she added: "Make no mistake, this time around Scotland's future will not be bought and sold for anyone's gold."

The SNP Government has proposed the two main campaign organisations, Yes Scotland and Better Together, should be allowed to spend no more than £750,000 in the crucial last 16 weeks of the campaign.

On top of that it also wants a spending limit of £250,000 for each political party represented at Holyrood during the so-called "regulated period" when strict rules apply.

However, the Electoral Commission has suggested a figure closer to £1.5m for each of the two main groups.

It has also advocated a spending formula for political parties based on their share of the vote at the last Holyrood election.

SNP chiefs fear the plan would limit them to £1.5m, while the pro-independence Greens would have £150,000 to spend.

Of the pro-UK parties, Labour would be allowed to spend £1.2m, the Tories £900,000 and the LibDems £600,000, giving the SNP's opponents a potential advantage of just over £1m.

Speaking after the signing of the Edinburgh Agreement, the deal between the Westminster and Holyrood governments paving the way for a legally watertight referendum, both Ms Sturgeon and Alex Salmond were at pains to stress the Scottish Parliament would have the final say on spending limits.

Last night, a source close to Ms Sturgeon went further, saying: "I don't think there was ever a suggestion it [abiding by the Electoral Commission's advice] was guaranteed.

"That's not in the agreement. It's the Scottish Parliament that is making the rules for the referendum."

Under the Edinburgh deal, the Scottish Government agreed to consult the commission on a wide range of campaign issues, including the wording of the question. The move was seen as a victory for the UK Government after SNP ministers originally proposed setting up their own body to oversee the referendum.

Scottish Secretary Michael Moore, who negotiated the agreement, said last week that his expectation was for the Nationalists to abide by Electoral Commission advice.

Ms Sturgeon's remarks provoked a furious backlash from the pro-UK Better Together campaign.A spokesman said: "We have the clearest signal yet that the rules for the referendum will be cooked up in Alex Salmond's front room.

"He cannot be a player as well as a referee on the same pitch. People in this country value fairness and they know when someone is at it."

Labour's Shadow Secretary of State for Scotland, Margaret Curran, said: "It didn't last even a week before the SNP decided to move the goal posts.

"No Government has ever gone against the Electoral Commission's recommendations and if the SNP doesn't accept its decision on spending limits in the referendum, then it will be an insult to Scottish democracy. If they want to have a credible process, then Nicola Sturgeon and the Yes Campaign need to give a clear commitment immediately that they will abide by what they are told by the Electoral Commission.

"There should be no ifs, no buts, no maybes or even room for a hint of a doubt: They need to accept that what the Electoral Commission says is final."

Scottish Tory deputy leader Jackson Carlaw said: "We know the SNP will attempt to rig this referendum in any way it can.

"Nicola Sturgeon may pay lip service to the Electoral Commission, but they are the ones who should set the rules on this, not the SNP."

Earlier the party hailed the first positive poll finding for some time with a survey of 972 voters by Panelbase showing a narrower gap than other polls – 45% No to 37% Yes. It revealed that, if Scots fear another Conservative-led Government at Westminster, support for independence rose to 52% with 40% voting against.

The same poll put the SNP at 45% in the party standings, 12 points ahead of Labour and on a par with last May's Holyrood result.