The Alliance Party leader, Mr Sean Neeson, has resigned, claiming his decision coincides with the ending of a phase in the peace process.
There had been speculation for some weeks that Mr Neeson had been considering his position following the party's poor performance at the polls.
He informed the Assembly party yesterday of his decision and said after three years it was time for a "fresh face" to take over the leadership. The Minister for Foreign Affairs, Mr Cowen, last night commended Mr Neeson for the "positive and valuable role" he played in the process.
"Those of us who have enjoyed working with him know how dedicated and committed he is to the causes of peace, partnership, equality and nonsectarianism," Mr Cowen added.
Mr Neeson (55), yesterday said the political process was entering a new phase, with the imminent suspension of the Assembly likely as difficulties continue with implementing the Belfast Agreement.
"There is a major dearth of ideas on how to break this deadlock," he said. "Every one at the Assembly believes that we'll go into suspension on September 22nd. So I think now is a prudent time to step aside." He said recent scenes of violence highlighted the need for the "cross-community and anti-sectarian voice of Alliance".
Mr Neeson will remain in place until a successor is elected at a special meeting of the party council on October 6th. Likely contenders include Assembly members Mr David Ford and Ms Eileen Bell, who was appointed deputy leader last Saturday.
Mr Ford said he was grateful to Mr Neeson for the job he had done in a difficult political climate, as the party suffered during the emergence of a "four party system". He declined to confirm he would be a candidate.
Mr Seamus Close, a deputy leader for 10 years, who resigned last June in protest over key issues, including the decision to step aside in a number of Westminster constituencies, may also contest the position.
Mr Neeson was appointed party leader following the decision of Lord Alderdice to become the presiding officer of the Assembly. Yesterday he admitted the public row between Mr Close and Lord Alderdice over this caused "major problems" for the party. "I would have preferred if all six of our Assembly members had worked from the floor," he said.
Between the 1997 and 2001 Westminster elections the Alliance share of the vote dropped from 8 per cent to 3.6 per cent.