Edinburgh's deputy lord provost has defected from Labour to the Scottish National Party, leaving Labour with no overall majority.
The defection leaves Labour with no overall majority
Councillor Steve Cardownie blamed what he called Labour's "lurch to the right of British politics" for his decision.
Scottish Labour said Mr Cardownie had "jumped ship before he was pushed" and the council leader called on him to stand down as deputy lord provost.
Labour is now dependent on the casting vote of Lord Provost Lesley Hinds.
The party now has 29 council seats, the Liberal Democrats 15, the Tories 13 and the SNP one.
Mr Cardownie, 52, has been a councillor in Edinburgh since 1988 and has been in the Labour Party since 1983.
He said: "With Labour's lurch to the right of British politics, the SNP is the only mainstream left-of-centre political party remaining for Scotland.
"It is for this reason I have decided to leave the Labour Party.
"This was not some sudden conversion with a single root cause but a move I have been considering for some time.
"I have discussed my disillusionment with the Labour Party with friends and family, but I became involved in politics to make a difference to people's lives."
Mr Salmond said: "Labour is now a right-of-centre party which lost its political soul.
"Gordon Brown confirmed this in his speech to the party's conference, when he stated he was a Blairite, and if he took over as leader we could only expect more of the same policies under him.
"Labour is a party without confidence in itself or its own ideas."
A spokesman for Scottish Labour said: "With re-selections for councillors on the horizon it will surprise few people that he has decided to jump ship before he was pushed.
"This decision does not tie into the most recent polling data which shows Labour are the party on the side of hard-working families and the Scottish nationalists are locked into a downward spiral towards obscurity.
"The Scottish nationalists are the party of parochialism and isolation. Steve Cardownie will certainly get that in the SNP Edinburgh City Council group of one."
Council leader Donald Anderson called on Mr Cardownie to stand down.
Mr Anderson said he was taking advice on the steps that could be taken to remove Mr Cardownie from office if he refused to resign.
The ruling Labour group also said the defected councillor's role as festivals and events champion was now "untenable".