A veteran Scottish MP tasked with finding a consensual cross-party alternative to independence has described the Yes campaign as a "totalitarian regime".

Former Liberal Democrat leader Sir Menzies Campbell contrasted the "emerging consensus" among the various unionist parties and non-party institutions for further devolution with the SNP, who are "totally dismissive of the ideas other than their own".

Former prime minister Gordon Brown is making a speech on Scottish independence today ahead of the publication of Labour's devolution proposals at its conference later this month, while a working group is examining the issue for the Scottish Conservatives.

The Campbell II report, released today, draws together the arguments of the other parties on devolution.

But Sir Menzies would not be drawn on how quickly these powers could be delivered, insisting it takes time to build consensus on the right mix of powers for Scotland.

"Unlike the Yes campaign, the campaign against independence is not some totalitarian outfit," he said.

"There are a variety of contributions from a variety of sources, but what is emerging is a consensus."

He added: "We've seen the white paper from the Government, we've heard contributions from Gordon Brown, Ruth Davidson, Devo Plus, the Institute of Public Policy Research, and the STUC.

"I was asked by Scottish Liberal Democrat leader Willie Rennie to take a measure of these contributions to assess the extent to which a consensus has emerged upon which we can build the kind of change which Gordon Brown is apparently going to describe."

The Scottish Government has dismissed Sir Menzies' proposals, insisting that the Lib Dems have been promising Scotland home rule for more than 100 years but have failed to deliver despite a number of terms in government.

Sir Menzies said: "One would expect that from the SNP because their response to anything other than independence is to be totally dismissive of the ideas other than their own.

"The fact that this has been a proposal with long-standing justification does not mean to say that it's irrelevant.

"We now have the opportunity to put it into operation."

Sir Menzies pledged to deliver his devolution proposals "as soon as is practicable" after a No vote, contrasting this with the SNP's pledge to deliver independence within 18 months of a Yes vote.

"It certainly doesn't mean trying to cram into 18 months everything that would be consequent a decision to support independence," he said.

"The SNP would say if we were going to do it tomorrow we were late.

"Of course there is not a timeframe for achieving consensus, because these are matters where there would have to be sensible, rational exchanges of views."

Sir Menzies chairs the Liberal Democrat Home Rule Commission, which published its vision for home rule in a federal UK in October 2012, advocating a substantial transfer of financial and constitutional power to Holyrood.