BBVA has called in consultants PwC to aid its investigation into alleged spying on government officials and businessmen by a security firm that worked for the Spanish bank in 2004, in a case that has grabbed headlines and prompted the European Central Bank to express concern.

"It's very important that the investigation be carried out very thoroughly and as quickly as possible to minimise the possible reputational risk that it may carry for the organisation (BBVA)," de Cos said late on Monday.

Spanish media reported earlier this month that BBVA had hired ex-police chief Jose Manuel Villarejo and his firm, Grupo Cenyt, in 2004 as part of efforts to prevent a takeover bid led by construction company Sacyr.

BBVA executive chairman Carlos Torres sent a letter to bank employees saying he was shocked by the reports and would widen an internal probe into the matter.

Francisco Gonzalez, who was executive chairman at the time, is now honorary chairman.

Asked about Gonzalez's current role at BBVA, de Cos told reporters this was "an important element" in terms of reputation for the bank, without elaborating.

"This is why it is very important that the probe carried out by this organisation (BBVA) be thorough and as quick as possible."

Gonzalez has not made any comment on the case, and could not immediately be reached for comment on Tuesday.

Villarejo was arrested in November 2017 as part of a separate investigation into alleged corruption, which has not yet been resolved.

(Reporting by Sabela Ojea; Writing by Ingrid Melander; Editing by Mark Potter)