Colum Eastwood is to resign as leader of the SDLP on Thursday, BBC News NI understands.
Mr Eastwood became leader of the party in 2015 and is the MP for Foyle.
The Irish News first reported the decision that he is to announce he is stepping down at a press conference on Thursday.
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 & Mid Down, the overall general election result was regarded as disappointing for the SDLP, which lost substantial ground in South Down and Upper Bann.
Mr Eastwood has been leader of the SDLP since 2015.
Mr Eastwood is due to speak at a press conference at noon on Thursday.
It is understood that he will remain as leader until a successor is chosen by the party.
It is expected that a replacement will be named in time for the party’s annual conference in October.
Writing on social media, DUP leader Gavin Robinson wished him well and said Mr Eastwood would “continue to champion Foyle in Westminster as before”.
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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.
Before politics
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 they heard the news on Thursday morning and that it had “caught them off guard”.
Lee Mitchell said she hopes Claire Hanna takes over as party leader, saying she is “a very competent operator” and believes, if she were to take the reins, that it could improve the fortunes of the party moving forward – who have struggled electorally in recent times.
“I think another woman in charge never is a bad thing,” she says.
Phil Langman, from Liverpool, who has lived in the city for 26 years, said Mr Eastwood had been a fantastic leader for the party, but understands why he thinks now is the right time to step aside.
“I think after about nine years in charge you can understand it” he says.
“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 believe 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” as leader 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.
Analysis – ‘Good time to step down for burnt-out leader’
If the news still comes as a surprise to many, within SDLP circles there’s been talk for some time that Colum Eastwood was feeling a bit burnt out.
And with the Westminster election over and any further planned elections three years away, it seemed as good a time as any to call it quits and step down on his own terms.
He will keep the Westminster seat for Foyle that he won again in the recent general election and he’s expected to turn his hand to helping the party’s New Ireland commission he set up.
As for who replaces Mr Eastwood there’s only one obvious choice: his colleague on the Green benches, Claire Hanna.
Read more of Jayne McCormack’s analysis on this story here.