Prime Minister in pledge to give Scotland more powers on tax

He told delegates that making MSPs more responsible for the money they spend was the "right thing to do" - effectively committing the Conservatives to going beyond the transfer of tax powers outlined in the 2012 Scotland Act.

Under the legislation, Holyrood will become responsible for setting about half of income tax paid by Scots from 2016.

MSPs are also taking control of a number of smaller taxes, including stamp duty.

However, in an unequivocal pledge, Mr Cameron said: "The Nationalists want people to believe that this is the end of the line for devolution. This is wrong. Let me be absolutely clear. A vote for No is not a vote for 'no change'."

He said the Tories were ­committed to making devolution work "better still", adding: "Not because we want to give Alex Salmond a consolation prize if Scotland votes No but because it's the right thing to do.

"Giving the Scottish ­Parliament greater responsibility for raising more of the money it spends - that's what Ruth (Davidson) believes and I believe it too.

"Vote yes - that is total ­separation. Vote no - that can mean further devolution, more power to the Scottish people and their Parliament but with the crucial insurance policy that comes with being part of the UK."

Aides said his use of the word "can" was not an attempt to leave room to reverse the promise at a later stage.

The pledge came as the Scottish Tories continue to develop proposals for further devolution.

A commission led by Lord Strathclyde, the former Leader of the House of Lords, is due to report to Scots Tory leader Ruth Davidson in May. The other two main pro-UK parties, Labour and the Liberal Democrats, have already backed further tax-raising powers for Holyrood.

The LibDems want to make the parliament responsible for raising 60% of the money its spends, up from about 12% at present.

Labour will unveil detailed proposals next week but former Prime Minister Gordon Brown has put forward a tax package that would make MSPs raise 40% of their spending.

Opinion polls suggest more Scots favour a beefed-up Holyrood than either independence or the status quo.

Addressing party members at the Edinburgh International Conference Centre, Mr Cameron stressed the economic advantages to Scotland of being part of a major world power.

He also insisted the nations of the UK were bound by a shared history and values.

He spoke proudly of the ­heroism shown in the First World War by his great-great uncle, Captain John Geddes, of the 16th Battalion Canadian Scottish. He also paid tribute to Eve Muirhead, skip of Britain's Winter Olympic curling team, saying the whole UK had been "transfixed" by their bronze medal-winning exploits.

Despite Mr Cameron's comments, Blair Jenkins, chief executive of the pro-independence Yes Scotland campaign, said only a Yes would guarantee more powers for Holyrood.

He said: "For many months now the Tories, like their Labour partners in the anti-independence coalition, have been bandying around vague promises of more powers if only the people of Scotland vote No.

"The trouble is we have heard it all before and it is clear from what the Prime Minister said in his speech that he cannot and will not guarantee more powers for the Scottish Parliament."

Mr Cameron gave no details of possible extra powers for Holyrood. However, Professor Adam Tomkins, a member of Lord Strathclyde's commission, said any transfer of powers would stop well short of so-called devo max. He said that system - which would put Holyrood in charge of everything but defence and foreign affairs - was a "perversion" of devolution and would eventually split the UK.

He said: "That's a Nationalist solution, not a Unionist solution. It is designed not to deepen and strengthen the Union but to break it."

He said the commission would set "clear lines" about the proportion of tax Holyrood should set when it reports in May.