THE Scottish Conservatives will tomorrow unveil what they describe as a "radical plan" to give Holyrood new tax powers in the event of a No vote in the independence referendum.

The proposal is at the heart of a commission into devolution chaired by former leader of the Lords, Tom Strathclyde and making Holyrood more accountable for its spending.

Reports yesterday suggest David Cameron is ready to devolve complete control over income tax, meaning MSPs would be able to set higher or lower rates than the rest of the UK, and Holyrood would be responsible for raising 40% of its spending, against 15% now.

Some welfare payments, including housing benefit, would be devolved under the plan, but ­corporation tax would remain UK-wide.

The proposals are more radical than those recently proposed by Scottish Labour, though not as far-reaching as those of the LibDems.

Publication of the report means all three Unionist parties have offered voters more devolution in the event of a No in the referendum, but they have not agreed a common plan.

Polls consistently show the public favour more powers over the status quo or independence.

The SNP claims the three parties cannot be trusted to deliver any new powers for Scotland, citing the broken promises made before the 1979 referendum.

The Unionists point to Labour legislating for the Scottish Parliament and the Tory-LibDem Coalition passing the 2012 Scotland Act giving Holyrood more power on income tax.

The Strathclyde Commission was announced in March 2013 in a dramatic U-turn by Scottish Tory leader Ruth Davidson.

Despite previously insisting the new Scotland Act was a "line in the sand" for her, she backed new tax powers to help make Holyrood more accountable for the money it spends. A source close to Davidson said the Strathclyde Commission would "provide a radical plan to increase the taxation powers of the Scottish Parliament" and that the Tory leader wanted a "grown-up, mature" system in Scotland. The source said: "It's not good enough to have a parliament that spends our money but doesn't have to account for how it raises that money.

"All you end up with is a parliament that judges success on how much it spends, and then blames Westminster when it has run out.The buck needs to stop at Holyrood.

"The block to progress is now the SNP ... the SNP doesn't want devolution."

The LibDems have called for powers to be devolved to Scotland as part of a move to a federal system for the whole UK, including most domestic policy and tax powers.

Labour recently published plans for giving Holyrood more power over income tax.

Earlier this year, Davidson ruled out a cross-party Unionist plan before September 18.

The No side also argue civic society - and the SNP - should be involved in drawing up the next phase of devolution, rather than political parties arranging a fix between themselves.

SNP MSP Derek Mackay said: "Whenever the Tories have been in a position to prove their commitment to more powers they have been found wanting."