Martin Schulz has quit as chairman of Germany's Social Democrats with immediate effect, saying it is time for a "renewal" of the centre-left party.
The move came after he on Friday abandoned his plan to become the country's foreign minister in a new coalition government with Chancellor Angela Merkel's conservatives, following pressure from within the party.
He told reporters the party's leadership had unanimously nominated his choice for a successor, parliamentary caucus leader Andrea Nahles.
She will have to be approved by a party congress on April 22, he said.
The turmoil at the top of the Social Democrats has been a major distraction as its leaders have been lobbying for the party to approve the new coalition agreement with Mrs Merkel's conservative bloc.
It is being put to a ballot of the Social Democrats' 460,000 members.
If members reject the coalition, Mrs Merkel's only realistic options would be to form a minority government or seek fresh elections.
Mr Schulz said the coalition agreement was "70%" Social Democratic policy and urged party members to bring "the personnel debate to an end" with his resignation and concentrate on passing the deal.
"This is a good agreement," he said.
The party chose deputy leader and Hamburg mayor Olaf Scholz as the new interim head until a permanent chairman is elected in April.
Mr Scholz will support Ms Nahles at the party congress.