Tommy Sheridan has resigned as leader of the Scottish Socialist Party with immediate effect.
Tommy Sheridan has resigned as SSP leader
A source within the party confirmed that the Glasgow MSP had quit for personal rather than political reasons.
Mr Sheridan tendered his resignation to the party's ruling body, the national executive, and it was accepted.
BBC Scotland political correspondent Glenn Campbell said Mr Sheridan, whose wife is expecting a baby, was keen to shed the SSP's image as a one-man band.
Nominations are now open for the position of leading the party, which has six seats in the Scottish Parliament.
Mr Sheridan was the party's sole representative in the first parliament after becoming a list MSP for Glasgow.
A statement, which was posted on the party's website once the news became public, said: "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.
"For the first four years of the Scottish Parliament, Tommy battled alone as the sole MSP, for 6 years he has led the party and as a larger than life public figure has come under intense personal pressure and scrutiny.
"These pressures have recently been aggravated by changes in his personal circumstances which have been widely reported.
"Faced with these pressures Tommy has decided to take a step back from the frontline of political struggle."
The statement insisted that the party was not a "one-man band" and contained an abundance of talent and a range of expertise.
But it said of Mr Sheridan: "Whether it was on striking workers' picket lines or his championing of the abolition of the council tax, Tommy Sheridan brought to Scottish politics a unique voice that spoke up for those who had no voice in the elitist world of establishment politics."
It emerged last month that the SSP was expected to go £200,000 into the red by the end of the current financial year.
Mr Sheridan conceded then that the party faced "a potential deficit" but said it was determined to "come through this even stronger".