Opposition parties yesterday joined forces in an unprecedented attack on the Scottish Executive's plans to hold a referendum on independence.

Labour, the Tories, and the LibDems issued a joint statement accusing the SNP administration of "squandering taxpayers' money in pursuit of their own narrow and failed agenda".

The move came on the eve of the publication today of an executive white paper setting out the SNP's proposals on independence.

First Minister Alex Salmond will launch the document in Edinburgh this morning as the first part of what the Nationalists say will be a "national conversation" on Scotland's constitutional future.

But while Labour, the Tories, and the LibDems have said they are willing to debate the prospect of winning more powers for the Scottish Parliament, they insist they will not do so while an independence referendum is on the agenda.

They point out that two-thirds of the electorate voted for unionist parties at the election in May, making independence "a minority obsession that will do great damage to Scotland".

Annabel Goldie, the Conservative leader, went so far as to say her party would seek to vote down the white paper before discussing greater powers for Holyrood.

She said: "We will not support the white paper on independence and call upon the Scottish National Party to recognise the clear majority view in the Scottish Parliament and in the country and to abandon this costly exercise immediately.

"If they do not, then the Scottish Conservatives will seek the earliest possible opportunity to bring this issue before the Scottish Parliament and force a vote."

The statement, in the name of Labour leader Jack McConnell, Nicol Stephen, and Miss Goldie, was drafted over the weekend. It said: "We are deeply concerned that the proposals to be made by the SNP are only designed to drag the people of Scotland into a constitutional cul-de-sac. We will not give succour to those who want to end the union.

"Their objective is to end the United Kingdom whilst our parties want to see devolution success inside the United Kingdom. Our parties have differing views on how to reform, maintain and strengthen the union, but none of us want to see it ended."

The statement adds: "We are willing to enter into debate about the way in which devolution within the UK can best develop and we believe that colleagues in Westminster have a role to play in that debate."

The opposition party leaders are expected to meet again next week. A senior aide to Mr Salmond said it was "hugely significant" Labour, the Tories and the LibDems now recognised the need for the present constitutional arrangement to be changed.