Mr Martin Morgan, a former Belfast lord mayor and the SDLP's European Parliament candidate in June, is to leave politics.
Mr Morgan (37), told his party of the decision some weeks ago, which was accepted with regret.
A member of Belfast City Council for north Belfast, including Ardoyne, for the past 12 years, Mr Morgan did not stand for the Assembly in 1998 or last November and was not included in the delegation for the Leeds Castle talks.
Mr Morgan is understood to want to devote more time to his career as a social worker.
Local government elections are scheduled for next year and he will not defend his council seat. Mr Mark Durkan, the SDLP leader, said Mr Morgan was "a loss to frontline politics".
"Martin Morgan is someone I have a lot of time for as a politician and a person," he added.
Mr Alban Maginness, Mr Morgan's north Belfast colleague and himself a former lord mayor, said he regretted the decision but acknowledged that it was up to Mr Morgan and his wife Dympna to decide their future.
He stressed: "There is no political disagreement" between the SDLP and Mr Morgan and that the decision was announced well in advance of next year's elections.
Mr Morgan was well-regarded inside the SDLP and beyond. His decision to seek the party nomination for the European Parliament following Mr John Hume's retirement announcement was held to be courageous.
He is widely held to have emerged from the campaign with his personal reputation enhanced despite the fact that the party vote dropped by over 12 per cent since the previous EU election and Mr Hume's seat was taken by Ms Bairbre de Brún of Sinn Féin.