The Spanish authorities opened a case against the 54-year-old in June for alleged tax evasion during his time as Real Madrid boss.
It is claimed he did not declare revenue related to his image rights in 2011 and 2012 and owes the Spanish state nearly £2.9million.
Mourinho’s agent issued a statement at the time that denied this allegation and pointed out he paid nearly £23million in taxes during his stint in Spain, at an average tax rate of more than 41 per cent.
It also stated that he had reached a settlement agreement with the Spanish taxman in 2013.
While it is understood that Mourinho’s legal team were expecting Friday’s news from Madrid, it does appear to have taken his employers at Manchester United by surprise.
The case against the charismatic but controversial Portuguese is part of a wider crackdown by the Spanish authorities against tax fraud and several leading figures in football have been caught out.
Argentine stars Angel Di Maria, Javier Mascherano and Lionel Messi have all been convicted of tax evasion in recent years, while Neymar’s transfer to Barcelona was the subject of a major investigation and Cristiano Ronaldo is also currently fighting alleged tax fraud charges.