It was billed by Tory leader Annabel Goldie as the most important debate the Scottish Parliament has ever staged.

Holyrood's unionist parties were yesterday beginning an unprecedented alliance to drive towards the second stage of home rule.

Miss Goldie was pledging her party's support for a Scottish constitutional commission, stressing that the proposal was "a new watershed in Scottish politics" and "more important and bigger than any political party".

She was aligning with LibDem leader Nicol Stephen in supporting a Labour motion at Holyrood that called for a new version of the Scottish Constitutional Convention that drew up the blueprint leading to the devolution referendum in 1997 and the following year's Scotland Act.

Last night, the three unionist parties secured a comfortable majority, by 76 to 46, for the initiative, though it left numerous questions around funding, membership, time-scale, remit and whether the commission's findings would be put to a referendum.

It sparked heightened dispute with the Scottish National Party, which has moved away from its consensual approach to drawing non-nationalists into debating the constitution. The National Conversation' on Scotland's constitutional futures was launched in August, with Alex Salmond eager for his rivals to put forward a devolution option for inclusion in a referendum.

By yesterday, the SNP was claiming credit for changing the unionist parties' stance on more powers, while being hostile to a commission that considers Scotland's future within the UK and was intended to exclude nationalists.

The motion called for the establishment of an independently chaired commission to review devolution in Scotland and encouraged Westminster politicians and parties to support the initiative.

While calling for funding from the parliament's budget, the plan is for a commission "that would enable the Scottish Parliament to better serve the people of Scotland, that would improve the financial accountability of the Scottish Parliament and that would continue to secure the position of Scotland within the United Kingdom".

The debate follows on from a keynote speech last Friday by Scottish Labour leader Wendy Alexander, in which she challenged her own party at Westminster to back a wide-ranging review of devolution, saying Labour should not be made up of "unthinking unionists".

She had cleared this approach with Miss Goldie and Mr Stephen, preparing the ground for yesterday's highly significant Holyrood debate. That leads to a second round of six-way talks between the three Scottish party leaders and their colleagues at Westminster.

However, this historic shift in the debate has been overshadowed by the controversy over Ms Alexander's campaign finances and doubts over whether she can remain in post.

The LibDem and Tory leaders were forced to explain that their support was for a longer-term aim and was not to support the Labour leader through her troubles. Deputy First Minister Nicola Sturgeon led a series of SNP attacks on Ms Alexander over her funding investigation.

There was criticism last night from Graeme Smith, leader of the Scottish Trades Union Congress, who said it was wrong to set out plans for a constitutional commission without even discussing it with the outside bodies, some of which were crucial partners in the Constitutional Convention, set up in 1989 which published its devolution plan the following year.

The Church of Scotland was another prominent backer of that convention, and Morag Mylne, convener of its Church and Society Council, yesterday said it would be open to discuss the plans so long as it did not lose its party neutrality. It is expected to be debated at the General Assembly next May.

Trade unions were among those who funded the earlier convention, along with local authorities and the Rowntree Reform Trust. The commission plan now is for the parliament to fund it and the necessary research on tax powers, though the SNP's Tricia Marwick, who represents the SNP on Holyrood's corporate body, argued there was no budget available without harming the work of the parliament.

Ms Alexander opened yesterday's debate, saying there was a desire for change "in the context of the union, which we believe has the interests of Scotland at heart".

She claimed the SNP was "scared stiff" by the alliance.

Mr Stephen's speech welcomed the Tories into the constitutional debate, saying this was its first involvement in two generations.