It seized on extracts released ahead of her speech today in which she said most Scots took more out of the economy than they put in.

She claimed just one in eight Scots, 12%, was a "net contributor".

US presidential candidate Mr Romney was secretly taped telling donors he did not concern himself with 47% of voters whom he said paid no income tax.

The Conservatives said they stood by the figures – which they said came from the 2010/11 Government Expenditure and Revenue Scotland report.

But SNP MSP Kenneth Gibson said Miss Davidson should apologise. He said: "At least Mitt Romney only insulted around half of Americans, while Ruth Davidson believes almost 90% of Scots do not contribute to society. It is an outrageous slur."

Miss Davidson was to tell a fringe event at the conference in Birmingham today: "It is staggering that public sector expenditure makes up a full 50% of Scotland's GDP and only 12% of people are net contributors, where the taxes they pay outweigh the benefits they receive through public spending."

She was also due to add: "Only 12% are responsible for generating Scotland's wealth.

"I wonder how many of them work on public-sector contracts?"

A Tory source said the Scottish Conservative leader was not giving a message to voters, but was pointing out an"economic reality".