The Spanish state secretary of the treasury Jose Enrique Fernandez de Moya says the body will take "inspections we consider appropriate" after allegations of tax evasion were levelled against Cristiano Ronaldo.

Over the next three weeks, a consortium of 12 European newspapers is set to publish the findings of what is described as "the largest leak in the history of sport" under the banner Football Leaks.

As part of that, numerous media outlets have alleged that Real Madrid superstar Ronaldo and Manchester United manager Jose Mourinho avoided taxes using tax havens through structures set up by Gestifute, the company founded by their agent Jorge Mendes.

The claims have been strongly refuted by Gestifute, but Fernandez de Moya assured that the Spanish authorities will take the steps they consider necessary.

"I must say that the information that the Spanish Treasury handles is what we know out of the media, and what I can say is that the Spanish Treasury is absolutely professional and we will take the inspections we consider appropriate," he told Cadena COPE.

Asked whether Ronaldo specifically was being investigated, Fernandez de Moya replied: "As we do with the whole of Spanish people.

"The information emerged yesterday [Friday] and therefore, the information that we have is what has come out through the media."

In a release issued on Saturday (AEDT), Gestifute described the accusations against Ronaldo and Mourinho as "unfounded", insisting the pair are "fully compliant" with regard to their tax obligations.

The statement read: "Both Cristiano Ronaldo and Jose Mourinho are fully compliant with their tax obligations with the Spanish and British tax authorities.

"Neither Cristiano Ronaldo nor Jose Mourinho have ever been involved in legal proceedings regarding the commission of a tax offence.

"Any insinuation or accusation made to Cristiano Ronaldo or Jose Mourinho over the commission of a tax offence will be reported to the legal authorities and prosecuted.

"We always act with the highest level of professionalism in our relationships with our clients and the authorities in all those countries in which we operate.

"We will not hesitate to take legal action to defend ourselves and our clients over any untrue and unlawful allegations published about us."