The Labour Party TD for Dublin North, Seán Ryan, has announced that he will not contest the next general election.
Mr Ryan told his party leader Pat Rabbitte of his decision on Monday and informed his constituency organisation last night.
Both the Socialist Party with Clare Daly and Fine Gael will challenge to take the Labour seat, but Mr Ryan said last night he believed Labour would retain it.
He told The Irish Times that at the selection convention before the 2002 general election, he told the local organisation that if this Dáil ran for a full term, he was unlikely to seek re-election.
"The current Dáil is now more than halfway through its term and I believe it is now an appropriate time to inform the local organisation and the people of Dublin North of my decision."
He said he would urge the party to hold an early selection convention to choose a new candidate, and he would "continue to work as a public representative for the people of Dublin North and the Labour Party up to the end of this Dáil term."
Mr Ryan said it had been a great privilege to do so as a public representative.
Mr Ryan (62) was a member of Dublin County Council from 1983 to 1993. He was first elected to the Dáil in 1989, lost his seat in 1997 and became a senator.
He was re-elected to the Dáil in a 1998 byelection and has retained his seat since then.
He said Mr Rabbitte had expressed regret when he told him of his decision, "but he accepted my decision and my assessment of the situation".