Mr Hume is a strong advocate of European parliament
Former SDLP leader John Hume has announced that he is to step down as a Westminster MP and European MEP.
He said the decision had been taken on the basis of medical advice.
The Nobel Peace Prize winner and Foyle MP used a news conference on Wednesday to make the announcement.
Mr Hume, 67, has served as a member of the European Parliament since 1979.
He also said he would step down at the next Westminster election.
Mr Hume has been Foyle's MP since the constituency was created in 1983.
He said it was a very regrettable announcement to make as it was a position he very much valued.
"In retiring from my European work, the reason I am doing so is because of medical evidence and medical advice, and the large workload and the amount of travel involved in the European Parliament.
"I would like to express my deep appreciation and gratitude to the people of Northern Ireland and to the members of my own party - the SDLP - who stood shoulder to shoulder with me and give me their support over the last 25 years."
His replacement as the SDLP's European candidate will be announced at the party's annual conference later this month.
Mr Hume had put his name forward to stand for Europe in the June election, but has now withdrawn.
SDLP leader Mark Durkan described his predecessor as "a person of huge insight, influence and inspiration".
"After June, he will no longer be a pillar of the European Parliament - but he remains a tower of strength to all who believe in peace, decency, democracy and social justice."
'Helm of the party'
European Parliament President Pat Cox described Mr Hume as one of the parliament's "most outstanding, respected and longest serving members".
"His dogged commitment to non-violent conflict resolution has been steadfast and constant," he said.
The Taoiseach Bertie Ahern said John Hume's departure from the European Parliament would leave a gap in politics locally.
"He is such a prominent national figure and we are a small island," said Mr Ahern.
"We do not have too many figures from this island that would be known in the political sphere all over the world.
"John Hume is such a person."
Sinn Fein President Gerry Adams said Mr Hume's decision not to stand again for the European Parliament marked the end of an era.
"The SDLP and ourselves have a different analysis and different objectives. But it is to John Hume's credit that he worked with us in trying to find a peaceful resolution of the causes of conflict on this island and between Britain and Ireland," he said.
Democratic Unionist MP Nigel Dodds said unionists viewed Mr Hume with mixed feelings.
"They will remember his passionate advocacy at one time for a united Ireland and his green agenda obviously," he said.
"But on the other hand people recognise politicians throughout Northern Ireland have worked very hard, most of them, for the people that they represented so clearly there will be mixed views as to his contribution as there will be for others."
Mr Hume came close to beating DUP leader Ian Paisley in the last European poll five years ago.
Mr Paisley announced last month he would not be standing for Europe again.
Mr Hume quit the leadership of the SDLP in November 2001 citing health reasons, after being at the helm of the party from 1979.
He was one of the founder members of the SDLP in 1970 and became deputy leader in 1973.
In August 1999, Mr Hume was rushed to hospital while attending a conference in the Austrian resort of Alpbach.
He underwent emergency bowel surgery which meant a period of recuperation.
Mr Hume - who received the Nobel Prize jointly in 1998 with Ulster Unionist leader David Trimble - vacated his seat in the Northern Ireland Assembly in 2000.