A CENTRAL plank in Alex Salmond's economic case for independence last year has collapsed after new figures revealed Scotland is no longer wealthier than the rest of the UK.

Statistics published by the Scottish Government revealed gross domestic product per head, the main international measure of a nation's wealth, dipped 1.1 per cent below that of the UK over the first six months of the financial year.

It is believed to be the first time in 35 years Scotland has trailed the UK and shows the impact of the falling value of oil production to the country's economy.

The figures came as the University of Strathclyde's Fraser of Allander Institute revised its forecasts for economic growth downwards this year and next.

In a report, the think tank blamed weak productivity and poor exports for the slowdown.

The news that Scotland's GDP per head slipped below that of the UK will raise questions about the SNP's approach to a second independence referendum, should First Minister Nicola Sturgeon choose to call one in the near future.

Six months before the referendum, the Scottish Government used the national accounts to include Scotland in an international league table of wealthy nations.

Scotland was in 14th place, ahead of the UK in 18th, out of 34 developed countries.

Finance Secretary John Swinney and other senior Nationalists used the table to argue there was "no doubt that Scotland can more than afford to be a successful independent nation".

Yes Scotland launched a poster campaign based on the figures with the slogan: "What would you say to living in one of the world's wealthiest nations?"

The figures, which related to 2012, showed Scotland's GDP per head was 11 per cent higher than the UK as a whole.

But according to the latest national accounts, Scotland's GDP per head for the first six months of the current financial year was £14,213, compared with £14,371 for the UK, or 1.1 per cent lower.

The Scottish Government failed to update the OECD league table to include Scotland earlier this year, though had it done so Scotland would have remained above the UK.

Figures for 2014 showed Scotland's GDP per head was 2.1 cent higher than the UK's.

Scotland's performance compared with the rest of the UK has fallen dramatically since 2008, when GDP per head was 15 per cent higher.

John McLaren, an economist and honorary professor at Adam Smith business school at Glasgow university, said: "This looks bad but in reality it is not as dramatic as it looks because most of the extra North Sea money, which has kept Scotland's GDP per head higher than the UK's, goes overseas.

"However, as that is not the impression the Scottish Government has been giving, it makes it harder for them to make the sort of claims they have in the past.

"It shows the North Sea is not all that important any more."

Jackie Baillie, Scottish Labour's public services spokeswoman, said: "These figures are just another reminder of why the SNP should forget all about holding another independence referendum any time soon.

"This is further proof, if proof were needed, that Scotland is better off remaining as part of the UK.”

Murdo Fraser, enterprise spokesman for the Scottish Conservatives said: "These figures bring another pillar of the SNP's independence con crashing to the floor.

"If we had believed Alex Salmond and Nicola Sturgeon last year we would now be less than six months away from leaving the UK - with barely any oil revenues and a lower GDP per capita."

A Scottish Government spokeswoman said:

"Scotland has been consistently wealthier per head than the UK per for the last 35 years, and in the 12 months to June this year, GDP per head in Scotland was £28,514 – just £11 per person, or less than 0.04 per cent, lower than the UK figure.

"The fluctuation in GDP per head is due in part to global oil prices – but growth in Scotland’s onshore economy is forecast to pick up strongly in the years ahead."