Rato was Spain's economy minister from 1996 to 2004 and led the International Monetary Fund from 2004 to 2007.
Rato, who was Spain's economy minister from 1996 to 2004 and led the International Monetary Fund from 2004 to 2007, failed to declare "a little more than 14 million euros (15 million dollars) in earnings from 2004 to 2015 inclusive", the report by the tax authority's fraud investigation division (ONIF) said, according to El Pais newspaper.
Half of the undeclared income -- some 6.8 million euros -- was concealed deliberately in a tax evasion system involving several companies, it said.
Four firms -- two based in Panama, one in Britain and another in Spain -- "were used to hide revenues and financial holdings from tax authorities, and the beneficiary was effectively Mr. Rodrigo Rato", ONIF said.
The fraud investigation, which took two years to complete, was delivered to a Spanish judge in January.
Rato, once a star of Spain's rightwing Popular Party, is also awaiting the verdict in a high-profile embezzlement case.
He is accused of overseeing a system while at Spain's Caja Madrid and then at Bankia that helped him and 64 others also on trial to use funds on items such as parties and luxury shopping without justifying or declaring the spending.
Altogether, they allegedly spent 12 million euros between 2003 and 2012 -- sometimes splashing out at the height of Spain's devastating economic crisis.
Rato has denied any wrongdoing in that case, and told El Pais the current claims made by the anti-fraud unit contained "false or erroneous information" and that he would answer the recent charges.