The SDLP leader, Mr John Hume, has stepped down from his seat in the Northern Ireland Assembly.
At the start of yesterday's sitting, the Speaker, Lord Alderdice, said he had received a letter from the Nobel peace laureate confirming his decision.
Mr Hume is expected to be replaced as the Assembly member for Foyle by Ms Annie Courtney, the party's leader on Derry City Council.
A by-election will not be held provided the chief electoral officer approves Ms Courtney as substitute. Speaking after the announcement, the Deputy First Minister paid tribute to Mr Hume's work but insisted he was not withdrawing from politics.
"In places of influence around the world, John Hume will be continuing to realise economic, social and political opportunity for all the citizens of Northern Ireland," Mr Seamus Mallon said.