EIP, which already emerged as a possible partner for Plenitude in June, has hired Rothschild as financial advisor on the deal and is organising the financing for its offer, which could materialise in autumn, the sources told Reuters.

Key details of the transaction are still to be decided including the valuation of the unit, the sources said, adding the stake on sale could be between 10 and 15%.

EIP was not immediately available to respond to Reuters' questions, while Eni and Rothschild declined to comment on the issue.

Last year analysts were pointing to a value for the unit of around 8 billion euros ($8.7 billion) including debt, while in July brokerage house Jefferies indicated the valuation could have increased to 10 billion euros as Plenitude expanded its business, adding more clients and renewable capacity.

In its research, Jefferies said a sale of a minority stake could delay an initial public offering (IPO) of Plenitude, but would provide a useful reference value for investors.

Eni, which last year had to freeze Plenitude's IPO due to adverse market conditions, still aims to list the unit, Chief Executive Claudio Descalzi said in July, pointing to 2024 for a potential market debut of the subsidiary.

Plenitude generates power from renewables, sells electricity, gas and energy services to households and businesses, and is developing a network of charging points for electric vehicles.

The sale of a minority would bring fresh resources to help Plenitude expand its business and become more autonomous from the financial point of view, analysts said.

($1 = 0.9183 euros)

(Reporting by Francesca Landini; Editing by Keith Weir)