CANON Kenyon Wright, one of the founding fathers of devolution, has warned MPs against trying to dictate the terms of the SNP Government's planned independence referendum.
The former chairman of the Scottish Constitutional Convention said veiled legal threats over the historic poll were "unwise" and would play into the Nationalists' hands.
Citing the Convention's guiding principle – that the Scottish people have a sovereign right to choose their own form of government – Canon Wright added: "I have a word of caution for Westminster. If the Union's unwritten constitutional claim to have the last word is so inflexible that it proves incapable of recognising Scotland's constitutional claim, then you are in effect saying that the only way in which Scotland's people can be sovereign, is by independence.
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"Is that what you really want to say to the Scottish people?"
Canon Wright was responding to a report from Westminster's cross-party Scottish Affairs Committee which said Holyrood did not possess the legal powers to hold a referendum on constitutional change.
The committee's convener, Labour MP Ian Davidson, claimed efforts by the Scottish Parliament to legislate for a referendum would risk a court challenge and indefinite legal wrangling.
He urged the UK Government, which retains responsibility for constitutional issues, to offer Holyrood legal guarantees – known as a Section 30 Order – as soon as possible.
However, MPs are only willing to make a single question referendum legally watertight. The stipulation has already been rejected by the Scottish Government, which is keen to include a fallback option of extra devolution – sometimes referred to as devo-max – on the ballot paper.
Responding to the Westminster committee, SNP Strategy Minister Bruce Crawford said the terms and timing of the referendum must be decided in Scotland. He stressed that could include a "more powers" option.
Backing the SNP's position, Canon Wright said: "It is surely time for Scotland to stand firm, as we have done in the past, against any attempt by the British Government or Parliament to use its theoretical sovereignty to dictate the terms of reference of the referendum, or the question or questions to be put. These are for Scotland's Parliament to decide."
The retired minister, under whose leadership the Constitutional Convention completed much of the groundwork for the Scottish Parliament, also said he hoped to see an option allowing for greater devolution on the ballot paper, if it was "defined with clarity".
His preference is for a system of "secure autonomy," modelled on the relationship between Spain and Catalonia, which would enshrine the sovereignty of the Scottish people within a constitutionally overhauled UK.
He said: "I've two objections to the very expression 'devo-max'.
"For a start it sounds rather like a soap powder. My other more serious objection is that the very word 'devolution' confirms the sovereignty of Westminster, since power devolved is power retained.
"What I am really talking about is not a little bit more of the same but the idea of secure autonomy. But of course that has to be spelled out very clearly."
He said he expected to see written constitutions for an independent Scotland and also an autonomous Scotland before the two options were put to voters.
First Minister Alex Salmond has offered increasing encouragement to supporters of greater devolution as opinion polls have indicated declining support for his goal of independence.
A number of groups are exploring or championing competing visions of Scotland's future.
The Future of Scotland group, a loose coalition of charities, churches and trades unions, are keen on a second question but have so far failed to present a detailed version of devo-max.
A separate group are campaigning for "devo plus", a system of greater financial autonomy devised by the centre-right Reform Scotland think-tank, but do not want it to be included in the referendum.
Labour and the LibDems have both set up party commissions to decide policy on further devolution, while Prime Minister David Cameron has made a vague promise to hand Holyrood more powers if independence is rejected in 2014.
Patricia Ferguson, Scottish Labour's constitutional spokeswoman, said: "The SNP Government has a mandate to ask the people of Scotland if they want to remain in the United Kingdom or not and they should get on with it."