Party leader Iain Gray and other opposition leaders also demanded First Minister Alex Salmond “clean up his act” after long-standing complaints about extreme attacks by SNP supporters dubbed “cybernats” on the internet.

The row follows the resignation of Mark MacLachlan, who worked in Mr Russell’s constituency office in Dumfries, after it was revealed he had used his blog to make “inappropriate” comments about the personal lives of members of other political parties.

Its timing is embarrassing as it coincides with the publication today of the White Paper on an independence referendum – the centrepiece of SNP policy and at the heart of Mr Russell’s work as

constitution minister.

Both Mr Gray and Tory leader Annabel Goldie said yesterday Mr Salmond must apologise to Parliament.

A Labour Party spokesperson said: “We can confirm that this reprehensible matter has been passed to solicitors to determine what recourse there may be for our representatives who have been so clearly defamed.

“This is dirty politics and the SNP constitution minister must make clear what he knew of this smear campaign and when. The SNP must come clean on whether any of their other advisers, aides or researchers are involved in similar activities.”

Mr Gray, who said he had been a victim of the bloggers, said: “Alex Salmond must apologise to Parliament over the vile smear campaign orchestrated from the office of one of his senior ministers.

“It raises extremely serious questions about Mr Russell’s knowledge and awareness of his aide’s activities and how his office was being abused.

“Last May, Labour called on Alex Salmond to root out the nationalist bloggers and their lies against not just politicians but also journalists. He turned a blind eye to the problem and must now explain to Parliament why he did so.”

Ms Goldie has written to the First Minister urging him to apologise for the “poison” spread by an SNP employee and to carry out an internal investigation to identify the extent of the problem. Mr Russell said he had been “absolutely shocked” by Mr MacLachlan’s actions.

“There is no place for despicable, anonymous commentary of this sort in politics. It shouldn’t happen in any party in any way,” he said.

Asked if it brought into question his own judgment in hiring Mr MacLachlan, Mr Russell added: “Quite clearly he was not the person who should have been doing that job. I’m very sorry about this. It has let me down and it has offended a lot of people.”

Mr MacLachlan said: “I unreservedly apologise to all the people concerned, and to Mike Russell. In getting carried away with my comments, which were very wrong, I have let myself and my colleagues down.

In his blog, Mr MacLachlan alleged some Labour politicians got a sexual thrill from bullying women, branded a prominent Tory “the biggest liar in the Scottish legal system” and claimed a married Labour MSP visited gay cruising spots.

LibDem leader Tavish Scott called on Holyrood Presiding Officer Alex Fergusson to mount an investigation.