PROSECUTORS in Spain’s highest court are investigating former king Juan Carlos over allegations that he received millions of dollars kickbacks from Saudi Arabia during the construction of a high-speed railway there by a Spanish consortium.

The general prosecutor’s office called the investigation one of “undeniable technical complexity”.

Ordered by the Supreme Court, the probe will centre on developments after mid-2014, covering the second phase of the 2011 contract for the bullet train between the Saudi cities of Medina and Mecca.

The date marks the time when Juan Carlos, marred by several scandals, passed on the throne to his son, who became King Felipe VI.

By abdicating after nearly four decades on the throne, Juan Carlos lost the protection granted to the head of state by the Spanish constitution.

A statement from the prosecutor's office said that given the “institutional significance”, the investigation would be charged to one of the Supreme Court's chief prosecutors and three assistants.

Although the former king’s finances have been questioned in Spanish media for years, there have so far been no legal implications for the king emeritus.

Spanish legislators have rejected at least two proposals since the first allegations emerged in open parliamentary investigations.

As The National reported in March, Swiss prosecutors are also investigating an account operated for Juan Carlos to which millions were allegedly transferred by Saudi Arabia's late King Abdullah.

Also in March, Felipe renounced any future inheritance from his father and removed him from the royal payroll after reports that the current monarch had been named as beneficiary for some of that money.

Felipe denied any knowledge of the fund.