The SDLP deputy leader, Ms Bríd Rodgers, will not stand in next May's Assembly election, the party announced yesterday.
Ms Rodgers (68), who is Minister of Agriculture and Rural Development, is retiring from representative politics for "family reasons".
However, she will retain her deputy leader's position for at least a year after the next SDLP annual conference in November.
She said yesterday she wanted to devote more time to her family of six and her grandchildren "whom I never see".
Her decision, alongside that of her predecessor, Mr Séamus Mallon, not to seek re-election to Stormont signals the gradual replacement of the founding members of the party with younger members.
The SDLP chairman, Mr Alex Attwood, said yesterday that Ms Rodgers's Upper Bann seat would be successfully defended. That was a "racing certainty", he said.
Mr John Hume has already resigned his Assembly seat and has been replaced by Ms Annie Courtney. A selection convention has already been held in the Newry-Armagh constituency to choose Mr Mallon's replacement, but this has proved inconclusive and is being rerun.
Ms Rodgers became deputy leader at last November's annual conference when she easily beat her nearest rival and assumed the role alongside Mr Mark Durkan, who gained the leadership at that time.
She became involved in community action shortly after moving to Lurgan, Co Armagh, in 1960 from her native Gweedore in Co Donegal.
She is fluent in both Irish and Italian. She was active in the civil rights movement and became spokeswoman for the Civil Rights Association in the town.
She was a founder-member of the SDLP in 1970 and rose to chair the party in 1978, the first woman to do so in an Irish political party.
She was also general secretary from 1981 to 1983 and was appointed to the Seanad in 1983 by Dr Garret FitzGerald.
She served on Craigavon Borough Council from 1985 until 1993 and unsuccessfully contested Westminster elections in both Upper Bann and West Tyrone. She lost out to Mr Pat Doherty of Sinn Féin in Tyrone in the last Westminster poll in June 2001.
She accepted the agriculture portfolio when the current Northern Executive was formed in 1999.
Ms Rodgers has been widely praised as Minister for her handling of the foot-and-mouth crisis and was voted Channel Four's Politician of the Year as selected by Assembly members.