TOMMY Sheridan, the Scottish Socialist Party convener and seen as one of the country's most unassailable political leaders, dramatically resigned last night, citing unspecified personal reasons.

The Glasgow list MSP tendered his resignation to the stunned SSP hierarchy, and it was accepted. But the party's national executive said it was up to Mr Sheridan to go into detail as to what the personal factors were.

The 40-year-old announced recently that he and his wife, Gail were to have their first child.

Party rules mean there can be no move to elect a new convener until the next conference in February, but it will now be up to the other five MSPs in the group to decide their own arrangements.

He had been leader since the party formed in 1999.

A dapper, tanned and charismatic figure, Mr Sheridan was often mocked for supposed vanity, yet he was seen as an astute political operator who had skilfully managed the transition of his party from a one-man-band formed out of the rump of Labour's old Militant Tendency into a serious political force with a Holyrood block of six members.

But there was also a price to be paid for the party's ambition, with it sliding heavily into debt in recent months.

As national convener, Mr Sheridan pleaded guilty to the sin of political ambition, saying that as the SSP campaigned across several fronts on issues such as the war in Iraq, poverty at home and the long-running nursery nurses dispute, the party had over-extended itself financially. It also turned its party newspaper, the Scottish Socialist Voice, into a more regular publication, at a very heavy cost, adding to the party's financial woes.

As a (pounds) 200,000 deficit loomed, Mr Sheridan and Rosemary Byrne, his South of Scotland parliamentary colleague, were forced to take out personal loans of (pounds) 39,000 each to provide a guarantee for the party. In common with the party's other MSPs they already turn over a large part of their salary to the party, so the loan arrangement simply transferred this straight to the bank.