MADRID (Reuters) - Property companies Merlin (>> Merlin Properties SOCIMI SA) and Metrovacesa have signed an agreement to form Spain's largest real estate group by rental assets, according to a company filing with the stock market regulator.

The agreement has the backing of Metrovacesa shareholders Banco Santander (>> Banco Santander, S.A.), BBVA (>> Banco Bilbao Vizcaya Argentaria S.A.) and Banco Popular (>> Banco Popular Espanol SA) which will hold a combined 31.2 percent stake in Merlin after he merger.

The new group will have a net asset value of almost 5 billion euros (3.84 billion pounds)gross annual revenue of 450 million euros.

Metrovacesa's 2.1 billion euros of assets will be split between Merlin, subsidiary Testa Residencial and a new company controlled by Metrovacesa's shareholders.

Santander said in a separate statement that it would own 21.95 percent of Merlin and 46.21 percent of Testa Residencial.

The merger still needs to be approved by each company's shareholders and given the green light by competition regulators, the companies said.

(Reporting by Robert Hetz; Writing by Angus Berwick; Editing by David Goodman)