Prime Minister Tony Blair has dismissed a top businessman's support for the Scottish National Party as "self-indulgent and absurd".
Tony Blair is in Scotland to boost Labour's election campaign
Former Royal Bank of Scotland chairman Sir George Mathewson backed SNP leader Alex Salmond for first minister.
He said independence could liberate Scotland from a "dependency culture".
Mr Salmond described Mr Blair's attack on Sir George as "foolish". The prime minister was in Edinburgh to warn that separation could bankrupt Scotland.
Sir George, who helped make the Royal Bank of Scotland the world's fifth biggest bank, claimed Labour was creating a "fear culture" about independence.
In a letter to The Scotsman he wrote: "It's difficult to forecast the future, but I see no circumstance where independence would be a serious [economic] disadvantage."
He also accused the Labour and Liberal Democrat coalition at Holyrood of having a "lack of high quality leadership".
Dismissing Sir George's comments, Mr Blair told a business breakfast: "You talk to real businesses, talk to the types of people I have just been addressing, and look at the impact of separation on real businesses and real families.
"It's absurd to say that there is not going to be a cost and a penalty - particularly when the SNP have got unfunded commitments, proposals for a local income tax, and when our two economies are so closely integrated."
Mr Salmond hit out at the prime ministers comments.
"This is a very foolish remark indeed, an extraordinary personal attack from a rattled prime minister," he said.
"George Mathewson has contributed more to the Scottish economy than probably any other single figure over the past 25 years."
Mr Salmond added: "Sir George personifies the success in business and public service that we want to see for the whole of Scotland."
Sir George backed the SNP to take Scotland forward
Liberal Democrat MP Malcolm Bruce said it was "well known" that Sir George supported the SNP, and had campaigned for the Nationalists in 1970.
"It is unfortunate that the SNP spin machine is using certain sections of the media to present this as some sort of conversion, in order to score political points," said the MP for Gordon said.
Mr Blair was in Edinburgh and Aberdeen to launch the Labour campaign warning that without Britain, Scotland could "end up broke".
A Labour document warned that an independent Scotland with the SNP in power would have a financial "black hole" which would have to be filled by taxes equivalent to £5,242 per household.
On the same day the SNP unveiled its policy programme for government.
The party plans greater local health control and community sentences for minor offenders.
The Tories said the election must be about issues that really concern people.