Anti-corruption Prosecutors working for the Spanish Supreme Court have taken over the investigation into the country's emeritus king Juan Carlos I and the possible receipt of commissions for helping win a contract for Spain to build high speed trains in Saudi Arabia.
Some 80 million euros in commissions has been reported as part of the 6.7 billion deal for the prestigious AVE rail system in 2011.
A court report says there could be evidence against the king, who abdicated in 2014 in favour of his son, Felipe VI. The investigation has to be carried out in the Supreme Court due to the king's protected status and the investigation will only cover activity from June 2014 onwards, as before that date the former monarch had immunity.
The suspicion started to fall on Juan Carlos after a former lover, Corinna Larsen, was secretly recorded saying that the king had received commissions.
As well as the investigation in Spain, Swiss authorities have been investigating possible money laundering and Corinna Larsen has launched a private case against the king for possible harassment.
In March this year, King Felipe publicly renounced his rights to any inheritance from his father.