Documents detailing secret government plans in the 1970s to prevent Scotland laying claim to North Sea oil have been seen by The Times. They show the extraordinary lengths to which civil servants were prepared to go to head off devolution, which was seen then as inevitably leading to independence.

The proposals included suggesting to Labour ministers, for whom devolution was a manifesto commitment, that progress towards a referendum should be delayed, in the hope that enthusiasm north of the Border would wane.

Treasury officials also advised that the boundaries of Scotland's coastal waters should be redrawn and a new sector created to “neutralise” Scotland's claim to North Sea oil – a step that was taken.

One Treasury official even proposed that a local campaign for independence in Orkney and Shetland should be encouraged so that Scotland would be denied access to more than half the North Sea oil. The idea was that the islands would prefer to throw in their lot with London rather than Edinburgh.

Among those advising Labour ministers was Sir David Walker, who is investigating the banking crisis for the present Government. As assistant secretary at the Treasury, he wrote in May 1975 that “progress toward devolution should be delayed for as long as possible consistently with honouring the government commitment to move down the devolution road and containing the SNP lobby in Parliament”.

Sir David's advice was heeded. It was another four years before the Scots were allowed to vote on whether or not they wanted an assembly in Edinburgh.

The documents – letters, memorandums and briefing papers from the Public Record Offices at Kew and in Edinburgh – show that some civil servants were alarmed by the threat that devolution posed to North Sea oil revenues, which were servicing Britain's external debt.

One paper, by Graham Kear, under-secretary at the Department of Energy, suggested that the Northern Isles might be hived off from Scotland. He wrote: “If Scotland and the Orkney and Shetland Islands are both regarded as states, separate from the rest of the United Kingdom, median lines can be drawn to divide the United Kingdom Continental Shelf between Orkney & Shetland/Scotland and between Scotland/England.”

One way of doing this, according to civil servants advising Anthony Crosland, the Environment Secretary, would be to realign the subsea border between Scotland and England, so that it ran northeast instead of east.

Mr Kear's doubts were shared by his political boss, Tony Benn, the Energy Secretary, who wrote to Ted Short, the deputy leader: “There is general agreement that energy policy – its formulation and execution – should be a function reserved to the UK Government.”

Mr Benn told The Times yesterday that he had favoured Scottish devolution. “I have always taken the view that power was too centralised,” he said. “I think you have to determine what it's appropriate to devolve. On the question of ownership of natural resources, that has to be seen as an integral part of the country.”

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