They opened an investigation into this in 2014 and on July 3, 2015 Mourinho was issued with a tax bill for €2.1 million ($3.1m) for the period 2010-2013 for that unpaid tax on image rights earnings of just under €12m ($17.7m).
According to the latest Football Leaks allegations he was able to reduce that tax bill by filing company expenses.
The investigation's revelations suggest Mourinho's image rights were ceded from a company owned by him called Koper, located in the British Virgin Islands, to two companies in Ireland.
Koper was in turn controlled by Kaitaia Trust, formed in Auckland in 2008. The investigation allegedly found the beneficiaries of the trust were listed as "Mourinho's current wife and his children".
Another document is said to state that, as of December 31, 2013, Kaitaia owed Mourinho nearly £10m ($17.6m).
However, Mourinho's agent Gestifute denied any wrongdoing on his part and said Mourinho paid more than €26m ($40.6m) in taxes when he lived in Spain from 2010-13.
Gestifute, which also has Ronaldo as a client, published documents allegedly showing that Spanish fiscal authorities have certified that Mourinho and Real Madrid star Ronaldo are "up to date on ... obligations."
Mourinho coached Real Madrid from 2010-13 and took charge of United this season.