MSP Tommy Sheridan has stepped down as leader of the Scottish Socialists with immediate effect.

The left-wing firebrand made his surprise announcement on Thursday saying that personal, rather than political, reasons had led him to the decision.

Sheridan was the sole SSP member of the Scottish parliament following its inception in 1999 but oversaw a dramatic rise to six MSPs in the 2003 elections.

He has also served several short prison terms for his part in anti-poll tax protests and demonstrations against the Faslane nuclear base.

By far the best known SSP politician, a party statement accepting the resignation acknowledged that Sheridan has at times appeared to be a "one man band".

"Tommy has played an outstanding role in the development of socialist politics in Scotland over the course of two decades and has become one of the most effective party spokespersons in UK politics," it said.

"For the first four years of the Scottish parliament, Tommy battled alone as the sole MSP, for six years he has led the party and as a larger than life public figure has come under intense personal pressure and scrutiny."

In a personal statement the 40-year-old former Labour Party member said he wanted to spend more time with his first child, which is due to be born next year, but would remain in Holyrood.

"I cannot give the time for a general election campaign and preparing for the 2007 elections, because I'm going to concentrate on being a father.

"I am going to have to seriously curtail my activities outside Glasgow.

"I'm an MSP for Glasgow, as party leader I have to be a party leader for Scotland and that is a big problem."

Nominations will now open for a successor to be elected, with a "team leadership" taking control until a winner is revealed.

"For far too long this party has been unfairly tagged as a one-man band. That label is about to be buried forever," Sheridan said.