Tommy Sheridan protesting at Faslane alongside George Galloway

He spent six months in jail in 1992 for defying a court order banning him from a warrant sale in Glasgow.

While a prisoner there he was elected as councillor for Pollok.

He was later jailed for refusing to pay the poll tax and for refusing to pay a fine after taking part in blockades at Faslane nuclear submarine base.

The fact that he was prepared to go to prison in defiance of the poll tax remains his strongest claim as a radical.

From an early age Sheridan was steeped in politics at home; he is undoubtedly a chip off a particularly hardened socialist block.

His mother Alice, who last year served two short terms in Cornton Vale for non-payment of poll-tax fines, brought her three children up in a house filled with biographies of famous trade unionists.

The colourful campaigner was educated at Lourdes Secondary School in Glasgow before going on to attend Stirling University, where Jack McConnell was a fellow student.

It was there he first became involved in left-wing politics, joining Labour's militant wing.

Notoriously outspoken

He said he joined the Labour Party "to make a difference", but by 1989 he had been expelled for his high-profile stance against the poll tax.

A spell as a councillor in Glasgow followed, before he founded the Scottish Socialist Party in 1999.

He cites becoming the party's national convener and being elected as the first socialist MSP for the region of Glasgow as one of the highlights of his career.

Since he was first elected to the Scottish Parliament five years ago, the SSP has grown to be the fifth biggest party in Scotland, with more than 60 branches.

Now 40, Tommy Sheridan has not mellowed with age.

He has backed campaigns including scrapping the council tax and most recently the plea by Rose Gentle, whose son Gordon was killed in Iraq, for Tony Blair to pull British troops out of the war-torn country.

Mr Sheridan's resignation for "personal reasons" will allow him to spend more time with wife Gail, who is expecting the couple's first child.

He said: "I am so busy with party business I am hardly ever at home but that has to change because I am going to be a dad."