Ieuan Wyn Jones is to resign as leader of Plaid Cymru after his party's poor performance in the Welsh assembly election.
Mr Jones has confirmed that he will step down, a week after Plaid suffered major losses in the poll.
It is believed that members of the Plaid Cymru group in the assembly were proposing to raise the issue of his leadership at a special executive meeting in Aberystwyth on Saturday.
It is also believed there may be tough questions about the handling of the election campaign by Jocelyn Davies, the AM who was Plaid's campaign manager.
In a statement released at 1700 BST on Thursday, Plaid announced that he would quit as both president and leader of the party group at the assembly.
Party chairman John Dixon paid tribute to him.
He said: "It is with great sadness that I have received news that Ieuan Wyn Jones is to stand down as president of the party and leader in the assembly, and I will be reporting on that to the meeting of the national executive on Saturday.
"On behalf of the members of the party, I'd like to thank Ieuan for his work as leader. I have personally seen how hard Ieuan has worked and how much personal effort he put into the recent election campaign.
"Ieuan will continue to serve as AM for Ynys Mon, and to contribute to the work of the party in the assembly, and I know that he will perform both roles with distinction."
Mr Jones will remain both party president and leader of the assembly group until the leadership election.
Plaid said the timetable would be discussed at this Saturday's meeting but it was thought the election would have taken place by the time of the party's conference in September.
Sources close to Mr Jones have indicated that he was shocked by this move by members of the Plaid group against him.
Mr Jones, who has been party president for less than three years, is due to make a statement at 1700 BST on Thursday.
Plaid's total number of assembly seats fell from 17 to 12 in last week's poll, a result which was was criticised as a "disaster" by his predecessor, Dafydd Wigley.
Mr Jones was elected leader by a large majority of party members in summer 2000, but ran into serious criticism about his party's alleged failure to safeguard the Welsh language in its so-called heartland areas in the north and west.
'I'm a fighter'
One of Plaid's MPs, Simon Thomas, had predicted the day after the last week's election that Mr Jones would face a challenge to his leadership.
But on the same day Mr Jones, the AM for Ynys Mon, issued a defiant statement declaring: "I'm a fighter, not a quitter."
Plaid could now be plunged into a divisive leadership contest, with no obvious contender ready to take up the reins.
First Minister Rhodri Morgan expressed his sympathy for Mr Jones.
Mr Morgan said the line between hero and zero was very fine and could happen to any politician.
He also said his only criticism was that Mr Jones could have chosen a better day from his point of view - as Mr Morgan had been announcing his cabinet throughout the day.
Leader of the Welsh Conservatives in the assembly Nick Bourne said: "I have great respect for Ieuan and I fully understand why he feels he has to relinquish the leadership of Plaid Cymru.
"Ieuan has been one of the major figureheads of his party and his continuing role and presence in the National Assembly for Wales will be to the benefit of the future of Plaid Cymru."
Plaid MEP Eurig Wyn, speaking from Brussels, said news of the statement had caught him "totally unaware", but it was needed to "clear the decks".
Mr Wyn said the groundswell of opinion tended to point to the view that the party needed a new central leadership.
Lembit Opik, leader of the Welsh Liberal Democrats, said it must have been hard for Mr Jones to make the decision.
"Perhaps it is at times such as these that we need to remember that most politicians enter the profession for the right reasons and this has always been the case for Ieuan Wyn Jones," said Mr Opik.