MADRID (Reuters) - Cellnex Telecom said on Thursday that it has agreed to buy Portuguese telecommunication tower operator OMTEL for around 800 million euros (£680 million) from Altice Europe NV and funds controlled by Morgan Stanley Infrastructure Partners.

The deal will be financed through existing cash reserves and should lift its adjusted earnings before interest, taxes, depreciation and amortization by around 90 million euros, the Spanish telecom tower operator said.

The group's sales backlog should rise by around 2.5 billion euros to 38.5 billion, Cellnex said.

Cellnex will initially pay 300 million euros for the company, with the remainder to be paid in 2027.

The acquisition marks Cellnex's first expansion into Portugal, its eighth European market, where it will take on Omtel's network of 3,000 cell towers.

Over the next four years Cellnex plans to install a further 400 sites around Portugal, it said.

Since its public listing in 2015, Barcelona-based Cellnex has been rapidly expanding its asset base, investing around 12 billion euros in acquiring or building up a portfolio of more than 48,000 sites.

Amsterdam-listed Altice Europe holds a 25% stake in OMTEL, while the remaining 75% is held by funds controlled by Morgan Stanley Infrastructure Partners.

(Reporting by Nathan Allen, editing by Inti Landauro and Emelia Sithole-Matarise)