The SNP has been “infiltrated” and “captured” by agents of the British state, a former MSP has said.

In a blog post on the Grousebeater website, Campbell Martin said his old party had been “completely compromised.”

“It was, and remains, insane to think that the British State would not have infiltrated the SNP,” he wrote. “A political party that claimed to have as its raison d’etre the break-up of the British State.”

The Home Office has so far refused to comment on the claim. 

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Mr Martin’s accusation comes just days after it emerged that the Security Service was able to identify the four young nationalists who liberated the Stone of Destiny in 1950 thanks to moles. 

Newly declassified documents revealed that struggling Special Branch detectives used Scottish-based informants to figure out who was behind the heist.

In his blog post, Mr Martin wrote:: “British State assets in the SNP have, over the intervening years, risen through the ranks and now hold senior positions that have allowed them to influence party policies and direction, such as adopting a lack of urgency in delivering independence…and that’s putting it mildly.”

He added: “The SNP is completely compromised, it has been captured and controlled by the British State.”

Mr Martin said the British State had “played a blinder.”

“Today’s SNP is so corrupted by British agents that it has sidelined independence and embraced gender policies that make the party unelectable. For the British State that is job done.”

Mr Martn was an SNP MSP between 2003 and 2004. He was expelled after criticising the leadership of John Swinney. 

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He said it was not unreasonable to suggest the infiltration of the SNP given the intelligence services managed to penetrate the IRA, the National Union of Miners, and the Socialist Worker Party. 

“It would be more extraordinary if they ignored the SNP, which is a party which supposedly wants to break up the British state,” he said. “If they hadn’t infiltrated that organisation there would be something far wrong.”

Jim Sillars, who served as the SNP’s deputy leader in the 1990s, is also convinced that British agents are operating within the party.

He told The Times: “When I joined in 1980, Dr Robert McIntyre [the party’s then president and first MP] took to me one side and said: ‘Do you realise we will all be penetrated’,” he said.

“In fact, there was some degree of suspicion that I might be one of the penetrators, given my Labour background. The great problem we have is identifying just how deep the infiltration goes and how high up it goes.”

Mr Sillars said he and his late wife, Margo MacDonald, the former SNP MP and independent MSP, “just accepted” that some supposed comrades were double agents. “There were a couple of people in the party who I was absolutely certain were MI5 plants,” he said. “There is nothing we can do about it.

“We are a constitutional national organisation and, therefore, we never set up a counter-intelligence department within the party.”

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Alex Neil, who served as health secretary in the Salmond government, and stood down as an MSP in 2021, also believes the security services are operating clandestinely within the Scottish government.

“If I were the first minister I would be operating on the assumption that there will be people who are out to ‘do us in’ from inside the organisation,” he said. “Who they are, who employs them, what they are actually doing, what positions they hold and how many there are, we just don’t know.

“I have no doubt that in decades to come, when official secrets are disclosed, it will emerge that the people who took back the Stone of Destiny were not the only ones who were betrayed by people they thought were loyal to the cause.”

The SNP and the Home Office have been approached for comment.