* Resignation to be formalised in two to three days - source
* Flavio Cattaneo favourite to succeed Patuano - sources
* Departure seen as sign of growing Vivendi influence (Recasts with details, Vivendi no comment)
By Agnieszka Flak and Danilo Masoni
MILAN, March 19 (Reuters) - Telecom Italia Chief Executive Marco Patuano is stepping down, four sources close to the matter said on Saturday, ending weeks of speculation about his future as top shareholder Vivendi tightens its grip.
Patuano's resignation will be formalised in the next few days, a fifth source said. Patuano, 51, whose mandate was due to run for another year, was not available for comment.
His relations with Vivendi have been tense since the French media company took a stake of just over 8 percent in Telecom Italia in June, a source close to the matter has said.
Vivendi got the stake as part payment for selling Brazilian broadband company GVT to Spain's Telefonica, which at the time was Telecom Italia's biggest shareholder.
Since then, the French company headed by tycoon Vincent Bollore has tripled its stake in Telecom Italia to 24.9 percent, secured four board seats and pressured Patuano to cut costs at home, and also decide what to do with its Brazilian business.
Sources have said Vivendi wants to sell TIM Participacoes , Brazil's second-biggest mobile network operator, while Patuano considers the business to be a strategic asset.
Vivendi declined to comment on Saturday.
Flavio Cattaneo, CEO of Italian railway company NTV and a Telecom Italia board member, is favourite to succeed Patuano, other sources, who declined to be named, told Reuters.
Cattaneo has many years of experience at state-controlled firms, including as head of Italian state broadcaster RAI and as CEO of gas grid operator Terna. He is also on the board of insurer Generali, where sources said Bollore exerted his influence in a recent management change.
A source close to Cattaneo said he had not received an offer from Telecom Italia and "is happy where he is". Cattaneo was not immediately available for comment.
The rest of Telecom Italia's board will stay on, with Chairman Giuseppe Recchi standing in as CEO until a replacement for Patuano is found, two sources said. Patuano's severance pay would be about 7 million euros ($7.9 million), one said.
Patuano, who started his career in Telecom Italia in 1990 and worked up through the ranks, has stepped up spending on faster, fixed and mobile networks as Italy seeks to get the country's internet up to speed with the rest of Europe.
He has pushed for asset sales to reduce Telecom Italia's debt of 27 billion euros and struck deals with content providers in search of new sources of income.
But industry insiders say the changes were not happening quickly enough for Vivendi and differences of opinion over the best Brazilian strategy were not helping.
Last month, Telecom Italia walked away from a proposal to merge TIM with rival Oi and one of the sources said a sale of the Brazilian mobile operator now looked likely.
"Patuano and the French had different views on Brazil. I now expect an acceleration on this front," said Tommaso Iaquinta of boutique investment bank Livolsi-Iaquinta & Partners.
Telecom Italia shares have risen about 20 percent during the past four weeks as investors bet that pressure from Vivendi could lead to a radical overhaul of the company, and possibly a tie-up with another telecoms or media firm.
The chief executive of French telecoms company Orange , Stephane Richard, said this month he would consider a tie-up with Telecom Italia if Bollore asked him but he did not think that was his intention.
Sources have also told Reuters that Vivendi is in talks to buy the pay-TV business of former Prime Minister Silvio Berlusconi's broadcaster Mediaset - a deal which could lead to a broader alliance between the two companies.
$1 = 0.8876 euros Additonal reporting by Paola Arosio, Maria Pia Quaglia, Valentina Za in Milan and Gwenaelle Barzic in Paris; editing by Greg Mahlich and David Clarke