The SNP's claim that Scotland would still be "one of the richest countries in the world" in the event of independence, has been dealt a blow by data showing North Sea oil and gas profits have sunk to a five-year low.

According to Office for National Statistics (ONS), the rate of profit at oil and gas exploration and extraction companies tanked to 27.6% in the first quarter of this year - its lowest level since the second quarter of 2009.

"Activity in the oil and gas extraction sector increased by 0.8% in Q1, however the industry has been in long term decline, contracting in every year since 2002," said the ONS in the report.

"The overall business environment was still weak but has been improving since mid 2013. Business investment grew by 10.6% in Q1 2014 compared to Q1 2013 however it still remains some way below (15.5%) the 2008 peak."

Scottish people will vote in an independence referendum on 18 September this year and will be asked the straight "yes/no" question: "should Scotland be an independent country?"

Scotland's Oil Revenues

Since the referendum period started on 30 May, the battle between the "Yes Scotland" camp and the "Better Together Campaign" has heated up.

Scotland's oil industry is regarded as the jewel in the SNP crown by leader and first minister Alex Salmond.

The National Institute of Economic and Social Research (NIESR), citing figures from the Scottish government, said around two thirds of all income from profits and employment due to the North Sea oil and gas industry were retained in Scotland to a tune of over £10bn (€12.5bn, $17bn) in 2010.

However, these latest figures show that Scotland's long term energy revenue predictions may not be as solid as Salmond claims.

The Organisation of Petroleum Exporting Countries (Opec) revealed that the average oil output in 2013 from the North Sea registered its lowest level since 1977.

This represented a roughly 10% decline from the previous year of 90 thousand barrels per day (tb/d).

Meanwhile, The Institute for Fiscal Studies (IFS) claimed that Scotland would borrow more money if voters decided to break away from the rest of Britain, as dwindling oil revenues and the short-term benefits of the UK recovery would fail to make it onto its balance sheet.