GOVERNMENT files on the Scottish National Party have been sealed for 50 years, fuelling accusations they contain evidence of a statebacked "dirty tricks" campaign to thwart independence.

A series of documents dating from the mid-1970s - created just as the party was about to suffer a catastrophic collapse in its vote - have been marked "closed or retained" because they are deemed too sensitive to release.

More than 30 files on the SNP created by Harold Wilson's Labour administration are held in the National Archives in Surrey, the main government repository of public records.

But while most are open to the public, it has emerged that a number covering a crucial period in the party's history have been withheld.

Four files from the Home Office, the department responsible for national security, have been closed under the 30-year rule.

One, however, headed Scottish National Party: General from 1974-1976 has had its contents sealed for 50 years - a decision that can only be taken by the Lord Chancellor and a special advisory committee when a file contains especially sensitive information.

Another retained file which was closed in the 1970s is entitled North Sea Oil: Scottish National Party.

The revelation that the government is holding secret files on the SNP last night sparked a major political row, with the party leadership in Westminster and veteran party members demanding the release of the documents.

It will also add fuel to longheld nationalist suspicions that Harold Wilson, under pressure from major SNP gains in the 1974 general elections, sanctioned the use of agent provocateurs to infiltrate and discredit the party.

Many nationalists active during the period have for decades laid the blame for the almost cataclysmic disintegration of the party's vote over the following years on the activities of the intelligence services.

Nationalist MSP Christine Grahame, who has been an SNP member for 35 years and whose office discovered the sealed files, said: "It is frankly outrageous that the state is withholding these documents. I am certain their reasons for doing so will be connected to a long suspected dirty tricks campaign which was waged against the party by British unionists who were frankly in a panic about the rise of the SNP."

Alex Salmond, the SNP leader, said: "One asks what the government has to hide in withholding the files. It does lead to the suspicion that the then Labour government was up to no good. They have no business keeping files on democratic political parties. I suppose it just tells you how paranoid the Labour government of the 1970s and perhaps its Tory predecessor were."

Rumours about agent provocateurs within nationalist ranks in the 1970s have raged for decades. It has been claimed that one such figure was Major Frederick Boothby, an ultranationalist who set up the 1320 Club - named after the date of the Declaration of Arbroath.

Boothby, who began recruiting young men to the extremist cause in the 1970s, published a magazine which contained instructions for bomb-making and began a terror group called the Army of Provisional Government, giving himself the code number 01 and the nom de guerre, Clydesdale.

Adam Busby, the founder of the Scottish National Liberation Army, was another recruited by Boothby in the 1970s. Busby, too, it is claimed, was working for Special Branch.

It is known that government and police agents were used within the trade union movement in the 1970s and, nationalists believe, similar tactics were employed to stop any further political gains by the SNP. By the end of the two general elections in 1974, the nationalists had 11 MPs and secured 30-per cent of all votes cast in Scotland. Labour, meanwhile, had just scraped a workable majority and the party in Scotland had suffered a major split.

In 1979 - only five years later - the SNP vote in the general election collapsed and the party spent a decade in the political wilderness.

Last night, however, political opponents accused the SNP of "paranoia". A Labour Party spokesman said: "The SNP appears totally paranoid. All the evidence shows they are absolutely no threat whatsoever to the British state."

A spokesman for the Scottish Conservatives said: "This is a typical knee-jerk response from a party that refuses to accept that it lost the argument and continues to hold a minority view."

A spokeswoman for the National Archives said she could not discuss why specific files had been closed, but added that documents were generally withheld because they contained sensitive information about national security or because they contained individuals' personal details.

A spokesman for the department of constitutional affairs, formerly the Lord Chancellor's department, said all requests under new freedom of information laws to open files would be "considered", but that sensitive files would remain closed.

liam. mcdougall@sundayherald. com