THE Yes campaign for independence is to refuse large donations from outside Scotland and from companies and trades unions – putting intense pressure on the No campaign to do likewise.

The SNP last night urged the pro-Union parties to sign up to the same voluntary rules in the interest of "fair play and transparency".

They proposed that for the first two years of the referendum campaign, which are otherwise unregulated, both sides should accept donations of more than £500 only from people on the electoral register in Scotland.

This would block donations of £500-plus from elsewhere in the UK, potentially choking off a major source of funding for the No camp.

Both sides have already agreed not to accept foreign donations of more than £500.

The No campaign, provisionally named Better Together, accused the SNP of trying to fix the rules on the 2014 referendum.

Besides underlining the message that Scotland's future should be decided by those who live here, the SNP's stance appears calculated to create a dilemma for unionists.

The stance on donations is a relatively easy one for the Yes Scotland campaign to take, as it already has £2m in its coffers, and its natural support is in Scotland.

It would be much harder for the cash-strapped No campaign to adopt, as it draws much of its support from outside Scotland.

Critically for the No campaign, which is being spearheaded by Labour, the SNP proposal would stop donations from unions during the two-year run-up to the ballot. A series of Scottish donors are reported to have declined to donate to the No campaign's funds, including Labour-backing tycoon Willie Haughey and the Tory fundraisers, Sir Jack Harvie and Lord Laidlaw.

In the absence of local funding, Labour should be able to call on its union backers, while the Tories can look to donors in the City of London.

But both sources would be out of bounds if the No camp followed Yes Scotland's lead.

However, rejecting the SNP plan lays the No camp open to the charge of outside interference.

Because the Scottish Parliament has yet to pass the legislation governing the referendum, there are currently no rules on spending or donations.

Only in the final 16-week "regulated period" before the vote will campaigns have to declare income and cap expenditure, and companies and unions register with the Electoral Commission as "permitted participants".

Until then, all ground rules are voluntary.

SNP campaign manager Angus Robertson said the new proposal would avoid the kind of funding rows that bedevilled the 1995 referendum on whether Quebec should secede from Canada, when a judge found the No campaign illegally spent around £400,000, then won with just 50.6% of the vote.

Robertson said under the SNP plan companies and unions would not be involved until the final 16 weeks, meaning only registered Scottish voters could donate more than £500 before mid-2014.

Calling for minimal "external input or influence", he said: "The campaign must be driven by those able to vote in the referendum. Above all, we want to see fair play and transparency.

"This referendum is about Scotland's future and it should not be unduly influenced by significant donations from those who won't have the vote. We hope those saying No to the people of Scotland having the political power to decide what's best for their own country will at least say Yes to committing to a fair and open campaign.

"We are working on the same basis as for UK elections, where it was decided, quite rightly, that only those on the electoral register should be allowed to donate to political parties. The same approach should be followed for this Scottish vote.

"There is a place for corporate and trades union donations, but these must be done on a separate and transparent basis and conform to the restrictions that will be put in place by the referendum legislation on permitted participants."

A spokesman for the No campaign said: "This is a pretty naked attack on trade unions by the Nationalists. We will abide by electoral law both before and after it comes into effect. The Nationalists should stop trying to fix the rules of the referendum."