Walter Veltroni ended a seven-year stint as mayor of Rome on Wednesday, stepping down in order to lead Italy's new Democratic Party to national elections in April.
After signing his letter of resignation, the 53-year-old politician was expected to formally announce his departure to city councillors during the afternoon.
Despite criticism from the centre right, Veltroni kept the mayor's job even after being elected Democratic Party (PD) leader at the end of last year.
He said he wanted to see a long-awaited blueprint for building and infrastructure in the capital approved before he left the helm.
It is still unclear who will take Veltroni's place although his predecessor in the job, Culture Minister Francesco Rutelli, is considering a return to Rome city hall.
Meanwhile, as PD leader, Veltroni forged an electoral alliance on Wednesday with the small Italy of Values party headed by former anti-corruption prosecutor Antonio Di Pietro.
The two parties will run under their own symbols but have agreed on a shared programme and Italy of Values supports Veltroni's candidacy to become Italian premier.
The PD leader's move was lambasted by the centre-right Forza Italia party of Silvio Berlusconi, who polls indicate as the likely winner of the April 13-14 vote.
''He's allying himself with a symbol of demagoguery and populism,'' said Forza Italia's Chiara Moroni. ''The Democratic Party has thus failed even before it has started''.
