Veteran socialist Eamonn McCann has announced he is stepping down from Derry City and Strabane District Council on health grounds.
The People Before Profit (PBP) councillor said he was resigning after being diagnosed with the neurological condition ataxia.
He said the condition has made "it increasingly difficult for me to fulfil my duty as a councillor".
Ataxia is a degenerative disease of the nervous system.
Mr McCann, a former MLA for Foyle, was previously diagnosed with epilepsy about 15 years ago.
"I have not found this disabling as far as being a councillor or a MLA was concerned. However, it was discovered during tests some months ago that I have developed a neurological condition, ataxia," he said.
"I have made my intention known to my party, People Before Profit, and have asked the party to set in motion the procedure for selecting a new councillor in my place."
Mr McCann was a leading figure in the civil rights campaign of the 1960s and early 1970s and is well known for his involvement with the trade union movements, especially in his home city of Londonderry.
He first stood for election in the Stormont general election of 1969, losing to a young John Hume.
After standing unsuccessfully for more than five decades, he was eventually elected in March 2016 at the age of 73 to the Northern Ireland Assembly as a People Before Profit politician.
He lost his seat the following year - the result of a snap election.
In 2019, he was elected to serve Derry and Strabane council in the Moor district electoral area.
Mr McCann said it has been "a privilege to represent the Moor DEA, where I was born and reared".
A journalist by profession, Mr McCann has had a career spanning more than 50 years during which he has written hundreds of articles and penned several books, including War in an Irish Town and What Happened in Derry, chronicling the events of Bloody Sunday.
Derry's Mayor Brian Tierney said he was "saddened" that Mr McCann had taken the decision to step down.
"As a veteran of the political scene, his vast knowledge and experience has given us all new insight into a wide range of local and international issues," he said.
"Eamonn's passion and his commitment to Derry and Strabane are at the forefront of all his campaigns, and council meetings will certainly be much less interesting without the colour brought by his intellect and rhetorical skill."