The North Sea could be set for a second boom as companies search for new sources of oil and gas to take advantage of record prices.

The popular view is that the UK's share of the North Sea is in decline, with energy reserves diminishing rapidly about 35 years after the oilfields were first exploited.

However, there is a growing body of opinion that suggests that proven oil reserves have been underestimated consistently.

Since the discovery of oil in the North Sea, the equivalent of 37billion barrels of oil have been extracted from the UK Continental Shelf, leaving up to 25.5billion barrels still to be recovered. However, industry experts believe that the remaining reserves exceed current estimates by as much as a fifth.

The claim that there could be almost as much North Sea oil left untapped as has already been brought ashore was made in a documentary, Truth, Lies, Oil and Scotland, screened on BBC One last night.

Alex Kemp, Professor of petroleum economics at Aberdeen University, said: ”We've produced, since day one, 37 billion barrels of oil. The remaining reserves on central estimates could be 20-22 billion barrels equivalent and on optimistic estimates could be over 30. So there still is a substantial amount left.”

Gordon Brown was yesterday urged to watch the programme by the Western Isles Nationalist MP Angus MacNeil.

During Prime Minister's Questions in the Commons Mr MacNeil said: “Can I give the Prime Minister another truth? My constituents in Lewis, Harris, Uist and Barra are paying the greatest fuel tax in the UK with fuel prices at £1.40 a litre - that's about £6.50 a gallon.

“Will he give some of the £4.4billion fuel windfall to offset fuel by 3 per cent in the Scottish Islands, something he has already agreed to do for areas of rural France?”

Mr Brown rejected the SNP's demands for a percentage of the extra duty on oil. He told the Commons: “Just as Scotland benefits from all the measures we've taken to deal with fuel poverty, so too North Sea oil is part of the revenues of the United Kingdom - and I will fight to defend the Union of the United Kingdom.”

The SNP leader Alex Salmond yesterday formally wrote to the Prime Minister calling for an agreement under which Scotland would receive a 10 per cent share of the £4.3 billion extra the Nationalists claim that the Treasury will receive from the higher oil prices.

The SNP also seized on claims made in the documentary that there may be up to 44 years' worth of oil left beneath the North Sea.

Peter O'Dell, of Erasmus University, in the Netherlands, said: “It's not quite as much as we've used already but it's not far short.”

New technology and the rising price of oil mean that it is now economically viable to drill fields once considered too difficult or too remote.

Richard Pike, chief executive of the Royal Society of Chemistry, argued in Petroleum Review this month that true proven reserves for the world may be nearly twice the conventional figure.

Mr Pike added that the current industry practice of reporting proven reserves alone was purely an historic convention that bore little relevance to what was actually produced.

There are many reasons why companies like to be conservative in reporting oil reserves, not least because it helps to maintain a high oil price. When Shell had to cut estimates by one fifth in 2004, it had a devastating impact on the company's share price and cost the members of the senior management team their jobs.

There are also concerns that if reserves are played up, politicians immediately set about calculating how much money they can get out of the oil companies.

Professor O'Dell's view that there will be new finds in parts of the UK Continental Shelf that have never been examined in any depth, would appear to be supported by the announcement last month that Dana Petroleum, a British independent company, found a new oilfield in the North Sea at West Rinnes.

The suggestion that the North Sea could harbour more oil than was previously forecast will cheer the Government, which made a surprise change last week to North Sea taxes, designed to boost falling investment levels in the UK Continental Shelf.

Investment in the shelf dropped by about £1billion in real terms to £4.9billion last year but much of the investment is coming from new entrants that are smaller and more dynamic than the behemoths of Shell and BP.