Andy Myles, a Liberal Democrat negotiator in the Scottish Constitutional Convention and the party's coalition with Labour, said there is no evidence that Scotland will become "a genuinely equal partner" with the rest of the UK if it votes No.
Lib Dem veteran Sir Menzies Campbell recently unveiled the party's latest proposal for federal "home rule", an ambition dating back to the premiership of William Gladstone in the late 19th century.
Mr Myles, who was special adviser to Deputy First Minister Nicol Stephen in the final years of the Labour/Liberal coalition at Holyrood, said he was "naive" to hope that Scottish devolution would lead to a British federation.
"After spending much of my adult life trying, genuinely, to improve government in the UK, I have come to the conclusion that there is a much better chance of bringing power closer to the people in an independent Scotland," he said.
"I campaigned for a devolved parliament because it brought power back closer to the people, and I thought it might shake-up the UK constitution and lead to major reforms. I now see that this was a naive hope or belief.
"I can see no evidence that it will lead on to a modern British federation, where Scotland is a genuinely equal partner with the other parts of the UK.
"None of the UK parties are even talking about what I consider to be federalism. I have come to the conclusion that the best way forward is an independent Scotland within the EU."
He added: "I want to live in an ordinary country, where the constitution is written down and largely accepted, but can be amended. I accept that such amendment should require very significant effort, but it should be possible.
"So I am forced in September to ask myself a simple question: Will I get to a written, amendable constitution quicker by going with the continuing UK or an independent, sovereign Scotland? I think that voting Yes is the surest way of getting to this benign position."
Yes Scotland chief executive Blair Jenkins said: "Gaining the support of Andy Myles is an illustration of the ever-growing appeal of a Yes vote to a wide cross-section of the community.
"Across the political spectrum, Yes now has backing from Labour people, Liberal Democrats, Scottish Greens, Scottish Socialists and even some Conservatives, as well as the SNP - and the thousands of others who, like me, have no party political affiliation and realise that placing Scotland's future in Scotland's hands is something we can, should and must do."