Mark Durkan has been the leader of the SDLP since 2001
SDLP leader Mark Durkan has said he intends to stand aside as party leader after the next Westminster election, which is expected around May next year.
Mr Durkan said he will defend his Foyle seat but does not believe he should continue to hold a dual mandate at the Stormont Assembly.
He made the comments on BBC Radio Ulster's Inside Politics programme.
Mr Durkan said he already indicated he did not want to continue as SDLP leader after his 50th birthday.
The 49-year-old replaced John Hume as leader of the Social Democratic and Labour Party in 2001.
He is still the youngest of Northern Ireland's main party leaders by nine years.
He said: "My position is that if I am elected again to Westminster as MP for Foyle I will be stepping down from the Assembly.
"I've said that openly on the record because I don't believe that that you can sustain dual mandates any longer."
Mr Durkan added that he believed Northern Ireland would be significantly impacted by cuts in public expenditure after the next election, and this had influenced his decision.
"If there is a Tory government elected, the implications in terms of the welfare system and other matters are such that I think there is more work to done at Westminster now.
"I think in that instance I need to be a full time MP for Foyle."
He also said that he did not believe that the SDLP, as a nationalist party, could "long be led from Westminster" but said he would to continue to play a strong leadership role in the party.
In a statement, the SDLP said the leadership change would be "addressed in a sensible and orderly fashion".
The party said it would also be putting forward "a number of new faces" in the Westminster election.
Mr Durkan took over from John Hume as SDLP leader in 2001, inheriting a party that was losing its place to Sinn Fein as the largest nationalist party.
He dropped out of his politics course at Queen's University in 1983 to work for Mr Hume, going on to become a Derry City councillor in 1993. He was SDLP chairman from 1990 to 1995.
After his involvement in negotiating the Good Friday Agreement in 1998, he was elected to the assembly.
He served as finance minister in the new power-sharing executive from 1999 until 2001, when he replaced party colleague Seamus Mallon to serve as deputy first minister until the assembly's suspension the following year.
In 2005, he became an MP, retaining Mr Hume's Foyle seat at the general election.