The inquiry is related to contracts with jailed ex-police chief Jose Manuel Villarejo, whom news websites El Confidencial and Moncloa.com reported in January the bank had hired to spy on executives of a potential buyer, construction company Sacyr, in 2004 when Gonzalez was BBVA's executive chairman.

In a statement on Wednesday Gonzalez said he was "willing to cooperate with the authorities" on the matter.

BBVA and the anti-corruption prosecutor declined to comment.

Spain's second biggest bank is already being investigated by the High Court on charges of bribery, disclosure of secrets and corruption in relation to this case.

BBVA, which has so far said it has not found any evidence of spying, is alleged to have hired Villarejo to also spy on leading politicians on behalf of the bank from 2004.

In March, Gonzalez temporarily stepped down as honorary chairman to avoid any harm to the bank's reputation during the inquiry.

Veteran banker Gonzalez, who is known in Spanish business circles by his initials "FG", was replaced by Carlos Torres as BBVA's executive chairman in January.

(Reporting by Jesús Aguado and Jose Elías Rodríguez; additional reporting by Jessica Jones; editing by Angus MacSwan and Giles Elgood)