Former Labour prime minister Gordon Brown has called for a written constitution which recognises the Scottish Parliament is "irreversible".
The idea was part of his vision for a Scotland that would stay part of the United Kingdom.
The people of Scotland will vote in a Scottish independence referendum on 18 September next year.
Scotland's First Minister Alex Salmond also made a speech in which he backed a written constitution post-independence.
He spoke to a gathering in Fraserburgh, Aberdeenshire, about what he wanted for Scotland's future.
The UK currently has what is referred to as an "uncodified" constitution - a set of rules taken from custom, precedent and legislation.
Mr Brown said he believed a written constitution could be used to highlight the importance of pooling resources across the UK.
The Kirkcaldy and Cowdenbeath MP made his views known at an event in Glasgow organised by United With Labour - the party's campaign to persuade people to vote against independence in next year's referendum.
Mr Brown insisted the SNP was trying to "frame the debate" about the country's future so independence was seen as the only positive choice and those who favoured remaining in the Union are "simply negative".
He added: "They want to create the impression the only debate in town is between their vision of independence and those people who oppose it.
"But we have a vision too, and we have a big idea. We actually have a bigger idea than the idea they are putting forward."
The former chancellor argued: "We pool and share resources and we do so so that we have equal economic, social and political rights for working people, for pensioners, for people in need of healthcare or unemployed people in need of a job, throughout the whole of the United Kingdom.
"I believe we should write this into the constitution, for the first time making it explicit that the purpose of the Union is not just defence security, is not just trading relationships, but to pool and share our resources for the benefit of working people, the elderly, children and families, in all parts of the United Kingdom.
"I believe it makes sense now, and is something we should have done in government, to write down in British law, that the purpose of the Union is to achieve these goals.
"I would also write in the British constitution that the Scottish Parliament is permanent, irreversible and indissolvable."
When the electorate go to the polls next year, they will be asked the straight yes/no question: "Should Scotland be an independent Scotland?"
At about the time Mr Brown made his address, SNP leader Mr Salmond expressed his desire to have a written constitution in the event of a "Yes" vote.
He first spoke in detail about the idea in February this year.
Mr Salmond said: "Like 16 other nations in the Commonwealth, we propose to retain the monarchy but the people of Scotland will draft a new constitution.
"The UK is currently the only country in the Commonwealth and the only country in Europe without a written constitution.
"A constitution is vital. It sets out the rights and liberties, protection of essential liberties of the people.
"I believe a written constitution can enshrine for all time the greatest Scottish tradition of all, the unshakable Scottish tradition that sovereignty lies with the people of Scotland."