By Andres Gonzalez and Corina Pons

LONDON/MADRID Morgan Stanley Infrastructure Partners is poised to buy Sacyr's Valoriza Servicios Medioambientales in a deal that could value the unit at close to 600 million euros ($644.8 million) including debt, people familiar with the matter said.

Morgan Stanley Infrastructure has prevailed over other firms in an auction that Valoriza's owner, Spanish construction firm Sacyr, held for the company which started in February, the sources said on Sunday.

Valoriza Servicios Medioambientales is specialized in managing a range of services in the waste recycling chain such as collection, treatment and management of waste.

If negotiations and financing arrangements are concluded successfully, a deal could be announced as early as next week, the sources added.

Sacyr declined to comment, while representatives for Morgan Stanley did not reply to a request for comment outside business hours.

The sources cautioned that there remained a chance that no deal would be completed, and asked not to be identified because the matter is confidential.

Sacyr has been advised by Santander and Nomura, while Morgan Stanley Infrastructure Partners was advised by Deutsche Bank, the sources said.

Sacyr, which also is seeking a partner for its facility services unit and its water subsidiary, has said it aims to use the funds from the sales to speed up growth in its concession infrastructure business.

($1 = 0.9305 euros)

(Reporting by Andres Gonzalez and Corina Pons; Editing by Jan Harvey)