Just days after the three No party leaders staged a public show of unity over more powers, Richard Keen QC attacked Labour's proposals as a failed and "horrible compromise" between the party's warring MSPs and MPs.
He also accused Labour of contributing to the "rise of the Nationalists in the West of Scotland" by becoming "thoroughly complacent" and taking voters for granted.
Yes Scotland said Keen's "unvarnished dissing" of Labour betrayed the "sheer depth of disunity" among the Unionist parties, and showed their promises on new powers could not be trusted.
Scottish Labour last night hit back. A party spokesman said: "Richard Keen's insights into Scottish politics are like Bernard Manning's tips on flower arranging. It's surprising he offers any at all, but ultimately we all know he doesn't know what he's talking about."
Keen's intervention comes as Labour, the Tories and Liberal Democrats try to convince voters a No vote would mean new clout for Holyrood.
Last week, the three Scottish party leaders, Johann Lamont, Ruth Davidson and Willie Rennie, publicly pledged more powers would be devolved from Westminster, regardless of who wins the 2015 general election. Their joint declaration said that although each party had its own "vision", their "common agenda" and "common endeavour" would see new powers on tax and social security devolved.
But while commending the Tories' proposals, Keen criticised Labour. In a briefing to journalists on Wednesday, he said: "I think that the Labour offering was incoherent, unworkable, confused and the product of a horrible compromise with the Labour Party at Westminster."
After first backing full devolution of income tax, Labour backtracked in March and instead proposed a complicated system which would give MSPs the power to vary rates by 15p and raise the top rates, but make it harder to cut tax. It was reported at the time that Labour MPs had watered down the plans, as substantial powers for Holyrood could mean a cut in their numbers.
The Tories have proposed full devolution of income tax rates and bands, with just the thresholds being set by Westminster.
Keen said if Scottish Labour had been left to its own devices it could have "come up with something that was at least coherent", but its tax idea was"slaughtered at Westminster". However, he insisted a deal on more powers was possible after a No vote, as all parties agreed Holyrood should be more accountable for its spending by raising more of its income.
One of Scotland's most respected lawyers, whose courtroom manner saw him nicknamed "Rottweiler", Keen became chairman of the Scottish Tories in January.
He said he felt Scots would reject independence, but the vote could be as narrow as 53 per cent to 47 per cent.
He also said only a "tiny minority" of people wanted independence regardless of the consequences, and compared hardline Nationalists to the Irish republicans of the Easter Rising.
"If you walked up to the Post Office in Dublin in 1916 and said to the guys inside, 'Listen, if you stay with Britain you will be £500 a year better off' they'd have shot you.
"They loathed the idea of being British, they loathed the English; the economic consequences were entirely secondary, if they came into their minds at all.
"That's what independence really and truly means, the desire to be independent full stop, and very, very few people in Scotland that I've ever met exhibit any such desire."
He also warned of a "very real danger of cronyism" in an independent Scotland, with those who backed a No shut out of jobs and contracts.
"No Irishman who fought with the British Army during the Second World War got a government job after their return to Ireland, not in the civil service, not in local authorities, not in the state-owned media, not anyone.
"That's an extreme example, but it's the sort of thing that can happen in a small jurisdiction."
He called recent online abuse of JK Rowling and other Unionists "political thuggery" and called on Alex Salmond to denounce the offenders, adding: "I'm not saying Alex Salmond condones it but he hasn't come out and condemned it."
If a Tory member made similar remarks, he would move to have them expelled, he said.
Yes Scotland chief executive Blair Jenkins said: "Only a few days ago we saw the leaders of the No parties making a joint promise of more powers for the Scottish Parliament in return for a No vote in September. Now we know from the chairman of the Scottish Tories this attempt to appear united was a complete sham.
"The truth is that the Tories, Labour and the LibDems cannot even agree among themselves; the chances of them reaching a cross-party agreement on more powers is nil."