ROME (AFP) - Italian leftist leader Matteo Renzi is to be nominated Italy's youngest-ever prime minister on Monday after a daring power grab that has been welcomed by investors but has left many Italians sceptical.

As head of the Democratic Party, Renzi engineered the ouster of his predecessor Enrico Letta - a member of his own party - accusing him of failing to live up to reform pledges in his stormy 10 months in government.

The 39-year-old will have to move quickly to form a new government once he gets the mandate from President Giorgio Napolitano as expected at 0930 GMT, and then deliver his promise to lead Italy "out of the quagmire".

The previous coalition of the Democratic Party, the centrists and the New Centre-Right party is expected to remain intact even though the leader of the latter, Angelino Alfano, has warned this is "not a given".

The New Centre-Right is a minor partner in the coalition but its votes would be critical for Renzi as all other parties say they are staying in opposition and he does not have a majority in parliament.

If he succeeds in forming the government, Renzi would then have to return to Napolitano for his nomination to be confirmed and would then be sworn into office.

Analysts say the whole process could take a few days.

The energetic and web-savvy Renzi, who is also mayor of Florence, has no previous experience in national government or parliament and is seen by many as having the right kind of outsider credentials for the job.

But his critics warn he risks burning out rapidly.

Many people seem willing to give him the benefit of the doubt even though they would have preferred early elections, as long as Renzi delivers on his promises to combat rampant unemployment and boost growth.