Tommy Sheridan's political party, Solidarity, appeared close to meltdown last night as its biggest internal faction, the Socialist Workers' Party, was forced to deny that it was on the point of walking out.

A lengthy critique of Solidarity's performance by the SWP was posted on a left-wing website "because of the significance of Ruth Black leaving Solidarity, on top of the arrest of Tommy Sheridan".

Councillor Black, the party's only elected representative in Scotland, this week defected to join the Labour group on Glasgow City Council.

Mr Sheridan was arrested last weekend and charged with perjury at his defamation case against a Sunday newspaper last year.

The damning assessment of the performance of the party, described as an SWP internal bulletin, spoke repeatedly about the SWP's relationship with the party in the past tense, implying that a split had already taken place.

But a spokesman for Solidarity insisted that the SWP had not left the party en masse and said that, while the document contained criticism, it did not amount to a schism.

"Solidarity is made up of several smaller groups and it is in their nature to have this kind of debate. Some in the SWP are miffed that this was leaked," he said.

In a statement put out late last night, Jimmy Ross, co-chair of Glasgow Solidarity and a member of the SWP's Scottish steering committee, reaffirmed the bond between the two groups.

He said: "There is no change in our relationship with Solidarity. We are committed to the project of building a left electoral alternative in Scotland and Solidarity is the only viable option for the SWP."

The SWP, as one of the bigger factions within the Scottish Socialist Party, had always fought an internal campaign against the SSP's pro-independence stance and when the SSP split at the time of Mr Sheridan's defamation case last year it left and joined the new breakaway Solidarity party. If it leaves Solidarity, this will be a hammer blow to Mr Sheridan's party as at a stroke it will remove a big proportion of its activists.

The critique of Solidarity's performance claimed that the party was "not participating fully in the broad and open campaigns that are the priority" and said that it could not lead a "new left formation" to fight against "New Labour, neoliberalism and reformism".

The statement added: "There is no other electoral alternative to Labour and the SNP in Scotland, and it is important that there should be one. We had hoped Solidarity could be that alternative."

Some of those who gave evidence in support of Mr Sheridan at his defamation hearing against the News of the World were leading SWP figures, so even the suggestion there is doubt about about their commitment to Solidarity is hugely damaging.

Mr Sheridan was arrested by police last Sunday as he left a radio station where he had being hosting a phone-in programme. At the same time he was being detained for questioning at a police station in Edinburgh and charged with perjury, officers raided his home in Cardonald and removed bags of evidence.

The Solidarity leader claimed that the way he was treated was vindictive and a waste of public resources.