Documents released by the Cabinet Office under the 30-year rule have been seized on by Nationalists.
North Sea Oil was in its infancy in 1975
A 1975 memo by Sir Kenneth Berrill, head of the central policy review staff, said Scotland could "go it alone" on the profits of North Sea oil.
SNP leader Alex Salmond said it showed "the extent of Whitehall's duplicity".
He said the memo admitted that a UK government would have to divert huge resources to Scotland for it to be as well off as under independence.
'No means secure'
The memo follows the release in October of a secret report written in 1974 which said that Scotland's oil revenues could have made a case for repealing the Act of Union.
The advice, prepared for ministers by economist Gavin McCrone, indicated that Scotland would prosper on the profits of North Sea oil.
The latest document was written by Sir Kenneth Berrill in reply to a Foreign Office memo which said the future of the Union was "by no means secure" and that devolution should be used to buy off calls for independence.
Sir Kenneth warned that increasing public spending for Scotland could anger deprived parts of England.
And he said it would be hard to fight the SNP's economic case for independence because "on fairly reasonable assumptions about the profits to be made from North Sea oil, Scotland could go it alone quite comfortably".
The SNP said other documents showed "most cynical" attempts to counter nationalist sentiment by exploiting tensions between mainland Scotland and Orkney and Shetland.
Mr Salmond said: "These latest devastating revelations demonstrate the full extent of Whitehall's duplicity and trickery in its desperate desire to cheat Scotland out of her oil and gas wealth."
"These latest secret documents confirm that UK policy towards oil in Scotland has always been motivated by smash-and-grab tactics.
"The focus is to grab as much of Scotland's riches as possible for Westminster and smash any claim by Scotland to a share of her own wealth."
He added: "The duplicity is quite extraordinary and the language of the most senior people in government regards Scotland as a colonial property to be divided and ruled by Westminster.
"The relevance for the politics of today is perfectly clear. Westminster is still engaged in smash-and-grab tactics, as the chancellor's recent oil tax announcement illustrates."