Angus Nicolson, a former SNP councillor and group leader on Western Isles Council, claimed Mr Russell asked him to get involved in an SNP blogging network.

Labour said it “blows apart” First Minister Alex Salmond’s claim that SNP “cybernat” bloggers were not officially sanctioned by party leaders.

As the row continued over the smears posted by Mr Russell’s former aide Mark MacLachlan on his site, the Universality of Cheese, Mr Nicolson wrote: “Back in the day when the SNP were more concerned with believing in the cause rather than absolute unquestioning devotion to the latest BlackBerry instruction of the day, I was contacted by Mike Russell to get involved in a pro-SNP blogging ring.

“I was to be part of the group as an influential blogger and I was to be contacted again as part of the blogging network he was setting up.

“Perhaps I was not pliant enough, but I never heard from Mike again. All of which explains why I blog in my own name and not under a nom de plume.”

Labour spokesman John Park said the allegations raised “very serious questions” for Mr Russell.

He said: “Who else did he approach to be part of the SNP’s blogging ring and did they include blogs that smeared political opponents? The Education Secretary is already under pressure to explain what he knew about the activities of his office manager who ran one of the most scurrilous Nationalist blogs.

“His position will become untenable if it turns out that he has misled the public.”

Mr Park claimed the “scandal” reached all the way to the First Minister’s office.

“His repeated claims that the so-called cybernats were not organised by the SNP have been blown out the water,” he said.

A spokesperson for Mr Russell said: “This is a nonsensical attack by Labour which is in itself an attempted smear of the type they claim to despise.

“Mr Russell has repeatedly made it clear that he had no knowledge of his former assistant’s anonymous blogging and unacceptable comments – for which the individual resigned.”

The spokesman said Mr Nicolson had been out of the SNP since March 2007 when he stated his intention to stand against the party in local elections.

Mr MacLachlan left Mr Russell’s employment after being exposed as the author of an attack blog which smeared politicians from other parties, called a Tory QC a liar and alleged a top Labour official bullied women.

He claims Mr Russell knew about the blog and did not resign but was effectively sacked and has started proceedings claiming unfair dismissal.

Mr Russell denies knowledge of the smears Mr MacLachlan was writing and has threatened to sue him for “totally false” allegations.