Alex Salmond's plans for a referendum on independence met a fierce barrage of criticism from his opponents yesterday.

Unionist MSPs rubbished the First Minister's proposal for a ballot paper in which voters would have to list their preferences.

It was made as he put pressure on those opposition parties to produce a clear plan to put to voters in a referendum that he wants in 2010. If they refuse, he warned he will make this a dominant theme of the following year's election campaign.

He also launched the second phase of his National Conversation on Scotland's constitutional future, appealing to an audience of civic leaders to take part.

If the newly formed Scottish Parliament Commission produces a clear plan for enhanced powers for Holyrood while staying within the UK, he said he expects three choices on the ballot form; independence, devolution with more powers or no change.

The Single Transferable Vote (STV) system, which was used for council elections last year, could mean the Scottish Government being given a mandate to negotiate independence on the basis of second choices, once the votes behind the least popular option are redistributed.

Tory leader Annabel Goldie said: "You do not decide the destiny of a country on the basis of the second best or least-worst option. This is tripe - the wild words of a panicking man."

Labour leader Wendy Alexander said: "Alex Salmond must be delusional if he thinks that Scotland will be happy to accept independence by the back door."

A spokesman for Mr Salmond said: "I don't think there is a more front-door option for the people of Scotland to choose their constitutional future than in a democratic referendum."

Liberal Democrat leader Nicol Stephen accused the SNP of trying to drag an unwilling Scotland to independence. "No matter how hard they push and prod, Scotland remains firmly determined to reject separation," he said. "This latest U-turn is a red herring, designed to deflect attention from the ever-decreasing levels of support for independence."

Mr Salmond lacks a majority of MSPs which he would need to secure support for an independence referendum. The invitation to add a devolution-plus option is intended to draw in support from opposition parties, and to avoid them jointly taking their eventual plan for more powers to a vote of the Scottish Parliament, providing a mandate for amending the Scotland Act at Westminster.

"Our preference is a straight choice for independence," Mr Salmond said. "But another way to do it if you've got another choice is 1, 2, 3. People are handling the idea of STV very well, so why shouldn't it be used? The idea that Scotland is not capable of engaging in a debate, crystallising choices and then making a decision, is nonsense."