Tommy Sheridan has said he resigned as leader of the Scottish Socialist Party so he can be a "proper father".
Tommy Sheridan and his wife Gail, who is expecting their first child
The 40-year-old politician and his wife Gail are expecting their first child next year.
Mr Sheridan also described gossip about his private life as "crap" and dismissed suggestions that he might want to seek a Westminster seat.
A statement by the party said there would be "collective" rule until a new leader is elected next February.
Branches now have until the first week in December to propose a new leader for the party, which has six seats in the Scottish Parliament.
The election will take place at the party's national conference and any member of the party is eligible to stand.
Mr Sheridan was the party's sole representative in the first parliament after becoming a list MSP for Glasgow.
He said on Wednesday: "I cannot continue to be the party convener and be a proper father. We are entering uncharted waters, Gail and I."
"We are older parents, and we are excited but petrified."
His resignation has surprised the Scottish political world and there is no obvious successor to take over the helm.
Mr Sheridan said: "Since the announcement of Gail's pregnancy, it has focussed my mind completely. We have been discussing this for some weeks."
He said he had intended to announce his resignation this Saturday, but his hand
was forced by a "false" media story that he was facing a possible leadership
Mr Sheridan added that the announcement now was to give the party time to
prepare to choose a new leader at its conference in February.
Mr Sheridan said: "If I'm going to stand down to try and make a fist of being a dad, I'm going to have to do it now.
"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.
SCOTTISH SOCIALIST MSPs
"Gail won't mind me leaving at 7.30am and getting back at six - but not at 11pm or midnight."
Mr Sheridan, whose own parents split up when he was a teenager, said he felt "relieved" at having made the decision and was looking forward to backbench life.
He added: "For far too long this party has been unfairly tagged as a one-man band. That label is about to be buried forever."
Asked about rumours concerning his private life, he said: "Gossip has attached itself to me for the best part of 20 years.
"It has always been crap and it remains crap, and that's got absolutely nothing to do with this decision."
Despite tendering his resignation on Wednesday, Mr Sheridan was in the Scottish Parliament's chamber on Thursday grilling First Minister Jack McConnell about troops in Iraq.
Wearing a white poppy on his lapel, he demanded that Scottish soldiers be withdrawn from the country as soon as possible.
A statement was posted on the party's website on Wednesday saying "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 added: "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."
The statement insisted that the party was not a "one-man band" and contained an abundance of talent and a range of expertise.
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".