UPDATE 4-Italy's Renzi to be sworn in Saturday after unveiling cabinet
* Centre-left leader names 16-member cabinet
* OECD chief economist Padoan named economy minister
* Confidence vote in parliament expected Monday
By Steve Scherer and James Mackenzie
ROME, Feb 21 (Reuters) - Italian centre-left leader Matteo Renzi promised on Friday to start work on reforms immediately, after he named a new cabinet and formally accepted the mandate to form an administration he said would stay in place until 2018.
He confirmed that OECD Chief Economist Pier Carlo Padoan, who was forced to hurry back from Australia, would take over at the economy ministry where he will play a central role in Renzi's bid to revitalise Italy's stagnant economy.
But there were few other big names in a 16-member cabinet dominated by relatively low-profile politicians with a sprinkling of non-political officials.
With a cabinet boasting no star names, the success or failure of the government will be down to the ambitious Renzi, who forced out party rival Enrico Letta last week after a stream of criticism over the slow pace of economic reforms.
At 39, he will be Italy's youngest prime minister and heads a cabinet made up mainly of ministers in their 40s and 50s, half of them women, continuing the rejuvenation of the elderly caste which used to run Italian politics.
Renzi, who became leader of the Democratic Party (PD) only in December, said his government would begin work immediately after being sworn in on Saturday.
"We're aiming to get started on things that need to be done from tomorrow morning," he told reporters after a two-and-a-half hour meeting with President Giorgio Napolitano.
But he will have to govern with the same cross-party alliance that hampered his predecessor's efforts. Six of his ministers were part of Letta's cabinet, three of them from the small centre-right NCD party on which his parliamentary majority depends, suggesting that he may have to tread cautiously at times to keep his coalition together.
Renzi, whose main experience in government has been as mayor of Florence, has sketched out ambitious plans for the eurozone's third-largest economy and said he aimed to stay in office until the end of the parliamentary term in four years' time.
Although he has provided few details, he has promised to tackle electoral and constitutional reform, make the labour market and tax systems more efficient and overhaul the bloated public administration all within four months.
With a fractious parliament and many ministers having to learn how to handle a complex administration as they go, the challenge is considerable.
"There's a need to move quickly on reforms but people forget that to get reforms done in Italy requires very specific skills and there's no guarantee that a good economist makes a good economy minister," said Alberto Mingardi, director general of the free-market think tank Istituto Bruno Leoni.
Despite his reputation as a fresh force out to break up the old structures that have held Italy back, Renzi will also be the third prime minister in a row to reach office without winning an election and does not even have a seat in parliament.
Although Italy's constitution does not require a prime minister to win a national ballot, opinion polls suggest many Italians are concerned about the lack of a mandate from voters, and questions about how he gained office could limit his ability to push through unpopular reform measures.
The country is only just showing signs of emerging from its longest slump since World War Two, fighting to hold on to a crumbling industrial base and provide jobs for millions of unemployed, many of them young.
Renzi's room for manoeuvre will be tightly constrained by the need to control Italy's 2 trillion-euro public debt, with the euro zone still scarred by memories of the crisis in 2011 which nearly broke the single currency apart.
Padoan, a respected former International Monetary Fund official, will be the fourth technocrat in a row at the economy ministry, the key contact point with the European Central Bank and European Union partners and an important factor in maintaining foreign investor confidence.
As head of the OECD's economics department, Padoan has called for aggressive easing from the European Central Bank and was an early critic of tough budget cutbacks in the euro zone's weakest economies as they struggled with excessive debt.
In other notable changes to the cabinet, NCD leader Angelino Alfano kept his post as interior minister, but will no longer have the title of deputy prime minister after Renzi ruled out giving him a post that could challenge his own authority.
Foreign Minister Emma Bonino, a well-known figure outside Italy, leaves the government to be replaced by Federica Mogherini, a 40-year-old defence and foreign policy specialist in the PD.
A poll on Friday by the SWG polling institute posted a dip in support for the PD, to 29.9 percent from 32.2 percent a week earlier, while support for former Prime Minister Silvio Berlusconi's Forza Italia rose to 21.8 percent from 20 percent.
The survey showed 27 percent saw Renzi as a leader capable of giving Italy a future, more than any other potential rival on the list. But that vote of support was still outscored by the 30 percent who picked "none".