Colum Eastwood has announced that he is to resign as leader of the SDLP saying he wants space and time to be a voice of change.
The Foyle MP said his decision to step down would "give someone else a chance" and re-energise the party.
He confirmed his decision to quit as party leader at a press conference on Thursday, after nine years in the job.
He said it had been a great privilege to serve as party leader.
Mr Eastwood will formally resign at the party conference on 5 October to allow the new leader to take up the position.
Announcing his resignation, Mr Eastwood said the "moment has come for me to stand aside".
He said: "Some people maybe don't realise how much effort you have to put in to the day-to-day running of a political party".
He said he wanted to focus on representing the people of Derry as Foyle MP, a job he will retain, as well as working with the New Ireland Commission, a body he set up to look at Irish reunification.
It is understood that he will remain as leader until a successor is chosen by the party.
Hanna tipped as successor
Asked about who his successor might be, Mr Eastwood said Claire Hanna, the South Belfast and Mid Down MP, would be "far and away the best option for members".
He said there was "absolutely no doubt" she had what it takes to be an SDLP leader.
"If this is my opportunity to be the first person to endorse Claire Hanna I'll absolutely take it. It's up to Claire but I hope she puts her name forward."
Political reaction
First Minister Michelle O'Neill said she wished the Foyle MP well, as he steps down as leader.
“We have worked together as leaders for many years now, to restore the political institutions in the north and in protecting the Good Friday Agreement and the all-island economy against efforts to impose the hardest possible Tory Brexit" she added.
Writing on social media, DUP leader Gavin Robinson wished him well and said Mr Eastwood would "continue to champion Foyle in Westminster as before".
The Taoiseach (Irish Prime Minister) Simon Harris also wished the former leader well.
"Colum has served with distinction and has been a progressive, responsible and pragmatic voice on all issues, but especially the challenging ones.
"As a leader he has been accessible, on the ground and close to his constituents in Derry and the issues that affected them" he added.
In a post on social media, Ulster Unionist Party (UUP) MLA Steve Aiken said: "Best wishes to [Colum Eastwood] as he steps down as leader.
"While there is much we have disagreed on, there is much we worked together on for the good of everyone Northern Ireland."
Secretary of State for Northern Ireland Hilary Benn said that Mr Eastwood had made a "great contribution" during his time as leader and said he looked forward to working with his successor.
A spokesperson for the Traditional Unionist Voice (TUV) said that Mr Eastwood's leadership "saw an increasing greening of the SDLP".
They added that "unionists will never forget that he infamously carried the coffin of an INLA terrorist".
Alliance Party deputy leader Stephen Farry said he had "enjoyed working together in areas where we could collaborate".
He said Mr Eastwood "has already made a significant contribution to politics and will no doubt continue to do so".
Tánaiste Micheál Martin said the "the north west of this island has had no more passionate a champion than him".
His resignation comes just eight weeks after he retained his Westminster seat with a reduced majority of 4,166.
While the party also retained its seat in South Belfast and Mid Down, the overall general election result was regarded as disappointing for the SDLP, which lost substantial ground in South Down and Upper Bann.
Who is Colum Eastwood?
At only 41 years old, Colum Eastwood may be considered by many a veteran of the Social Democratic and Labour Party
Mr Eastwood joined the party in 1998.
The 2015 party conference - where he was elected leader - was his 18th conference and he has held elected office for almost two decades.
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Youngest mayor
Born in Londonderry in 1983, Mr Eastwood was educated at St Columb's College in the city which counts two Nobel laureates - poet Seamus Heaney and long-time SDLP leader John Hume - among its former pupils.
He has said Mr Hume and his deputy Seamus Mallon were among the figures who drew him to the party to campaign for the Good Friday Agreement.
In 2005, just a week after his 22nd birthday, he was elected to Derry City Council and between 2010 and 2011 served as the city's youngest ever mayor.
That was followed in May 2011 by election to the Northern Ireland Assembly.
When did he become SDLP leader?
In 2015 he challenged incumbent Alasdair McDonnell for the leadership of the party, ousting him by 172 votes to 133.
His election as leader brought attention back to a controversial moment in 2012, when he carried the coffin of a personal friend at a paramilitary-style funeral.
He said he was acting in a personal capacity and defended his decision.
In the 2019 Westminster election, Mr Eastwood was attempting to win the seat back from Sinn Féin who had won it for the first time by a narrow margin at the previous election.
He emerged victorious with a majority of 17,110.
What do Colum Eastwood's constituents think?
Michael McBride, reporting from Derry
John and Lee Mitchell said they were both stunned to hear the news.
Both traditionally SDLP voters, they said the news had “caught them off guard”.
Lee Mitchell said she hoped Claire Hanna takes over as party leader, saying she is “a very competent operator” and could improve the fortunes of the party which has struggled electorally in recent times.
“I think another woman in charge never is a bad thing,” she said.
Phil Langman, from Liverpool, who has lived in the city for 26 years, said Mr Eastwood had been a fantastic leader, but understood why he thinks now is the right time to step aside.
“I think he’s a fantastic MP, he seems like a decent man, but it must take its toll fronting a political party," he added.
“I think anyone following in the footsteps of people like John Hume as leader will always struggle to make as much of a mark on politics here like he did.”
Trevor and Ann Porter said whoever takes over will have big shoes to fill.
Ann said the party had not been fortunate in recent elections, but still felt the Foyle MP was the best man to “steer the ship” in the years ahead.
However, one man, who did not want to give his name, said Mr Eastwood will not be missed.
“They are a dying party. I’ll absolutely not miss him,” he told BBC News NI.