The 33-year-old was accused last year of evading €14.7million in taxes relating to his image rights between 2011 and 2014.
According to reports in Spain on Monday, Ronaldo's legal team made a settlement offer of €3.8m, plus admission of wrongdoing, which was rejected by the country's tax authorities.
The Portugal international appeared in court last year to respond to the allegations, insisting "I always pay what I have to pay; I do things the right way and I will continue to do so". His representatives, Gestifute, have also persistently denied any wrongdoing.