Kgalema Motlanthe is an ANC veteran but only became an MP in May
South Africa's ruling African National Congress (ANC) has chosen a caretaker president to replace Thabo Mbeki.
Sources within the party have named deputy leader Kgalema Motlanthe, an ally of party leader Jacob Zuma.
Mr Zuma said on Monday the decision would only be formally announced when parliament meets - on Thursday - to vote on the appointment.
Thabo Mbeki resigned on Sunday over claims of political interference in a corruption case against Mr Zuma.
He denies the allegations but said he was stepping down at the request of the ANC in the interests of party unity.
In his first comments on the case, Mr Zuma said it was one of the most difficult decisions in the ANC's history.
But Archbishop Desmond Tutu said he was "deeply disturbed" by Mr Mbeki's forced resignation, saying the interests of the ANC had been put before those of the nation.
"Our country deserves better. The way of retribution leads to a banana republic," he said.
Mr Zuma, however, promised a smooth transition.
Zuma says Kgalema Motlanthe could be president
"This is not a change of party but only leadership in government," he told reporters.
"Our economic policies will remain stable, progressive and unchanged."
Mr Mbeki has said he will remain in office until his successor is chosen.
The ANC chief whip on Monday moved a motion for Mr Mbeki's resignation to take effect on Thursday.
The new president will hold the post until elections are held in early 2009, which Mr Zuma is widely expected to win.
Mr Zuma said the candidate would be named in parliament "at an appropriate moment".
However, he said of Mr Motlanthe - the man heavily tipped for the post - "I am confident that if given that responsibility he will be equal to the task".
If confirmed, it would be a meteoric rise for Mr Motlanthe, who only became an MP in May.
However he has impeccable ANC credentials, the BBC's Jonah Fisher in Johannesburg says.
Mr Motlanthe spent much of the 1980s jailed on Robben Island along with Nelson Mandela.
He was made ANC secretary general in 1997 and became the party's number two in December, at the same time as Jacob Zuma was elected party leader.
ANC General Secretary Gwede Mantashe said the decision to ask for Mr Mbeki's resignation was taken to avoid divisions in the party.
Mr Zuma, he said, would not take over immediately but would wait for general elections when, if voted in, he would be "a people's president".
Mbeki announces his resignation
Mr Mantashe insisted the resignation call was not a punishment for Mr Mbeki and that the president would be given the chance to continue his role as mediator in Zimbabwe.
However, the main opposition party, the Democratic Alliance, said Mr Zuma had "got what he wanted" from Mr Mbeki's resignation.
Party leader Helen Zille told the BBC's Network Africa programme that Mr Zuma wanted "a political solution to his legal problems".
Praising Mr Mbeki as a strategic thinker and peacemaker, she described Mr Zuma as "the leader of a rabble out to grab the spoils of state for their own interests".
Mr Mbeki's resignation came days after a high court judge suggested he might have interfered in a corruption case against Mr Zuma.
In his television address, Mr Mbeki made an impassioned defence of his position.
Neither he nor his cabinet had made any attempt to meddle with the judicial process, he said, and he dismissed any suggestion he had been trying to shape the judgement for his own political ends.
Mr Mbeki fired Mr Zuma as deputy president in 2005 after his financial adviser was found guilty of soliciting a bribe on his behalf.
But Mr Zuma returned to the political stage to topple his rival as ANC leader in bitterly contested elections last year.