Tony Blair yesterday claimed plans by the Scottish National Party would land each Scottish household with an extra tax bill of more than £5000 a year.

The Prime Minister unveiled one of the Government's main campaign weapons as the electoral battle between Labour and the SNP intensified ahead of this weekend's SNP conference in Glasgow.

With the slogan "Break up Britain - End Up Broke", Labour has updated its warnings against independence that saw it beat the SNP in 1999 and 2003. It is banking on the figure to boost fears among Scots that independence could be expensive and risky.

The analysis, much of it compiled by former Enterprise Minister Wendy Alexander, concludes there would be an £11.9bn annual deficit in an independent Scotland, and the political attack splits that evenly between Scotland's 2.27 million households: £5242 each.

The calculation is made up of a disputed reckoning of the gap between tax revenue and expenditure in Scotland. Added to that are SNP promises for both devolved and reserved policies, plus the costs of running separate institutions for Scotland, ranging from a tax collection agency to the armed forces.

However, on the day Mr Blair returned to the campaign trail in Edinburgh and Aberdeen, his party took a severe knock from the endorsement of Alex Salmond as First Minister by the man who built the Royal Bank of Scotland into a global company.

Sir George Mathewson, the bank's former chairman, yesterday dismissed as "patently absurd" Labour's claims about the potential damage independence could do to the financial sector.

In a reference to Jack McConnell and Labour's primacy at the Scottish Executive, he said Holyrood politically had "disappointed since its creation, partially due to the lack of high-quality leadership".

Mr Blair hit back at a press conference: "I regard it as pure self-indulgence. You talk to real businesses, 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 described the attack on Sir George as "an extraordinary personal attack from a rattled Prime Minister. George Mathewson has contributed more to the Scottish economy than probably any other single figure over the past 25 years".

Labour, which has struggled to match business and economists' endorsements for the SNP over recent weeks, suggested Sir George would never have made his comments if he were still answerable to Royal Bank shareholders. It was claimed by Labour that Sandy Crombie, chief executive of Standard Life, has concerns about independence.

However, contacted, Mr Crombie remained neutral.

Speaking in Edinburgh, Mr Blair stressed that it was important for Scots to "think carefully" before casting their ballots. Facing negative poll findings about him, he responded: "In the next few months, I'm off the political scene, I'm no longer Prime Minister. But if you elect an SNP government you are going to be living with them, and their commitment to independence and a referendum within the next parliament. You are going to be living with that for years. And at a cost - £5000 per household.