Spain's former king, Juan Carlos I, has written a letter revealing plans to leave the country, after corruption allegations surfaced in recent weeks.

The 82-year-old former king is credited with helping Spain peacefully restore democracy after the death of dictator Francisco Franco in 1975.

But marred by scandals in the later years of his reign, Juan Carlos in 2014 abdicated in favour of his son

The royal house has denied that Felipe had any knowledge of his father's alleged financial irregularities. In June, the Spanish Supreme Court opened an investigation into Juan Carlos' involvement in a high-speed rail contract in Saudi Arabia.

Switzerland's La Tribune de Geneve newspaper reported he had received $US100 million ($140 million) from the late Saudi King Abdullah.

Through his lawyer, Juan Carlos, 82, has declined to comment on the allegations.

The royal family's website on Monday (local time) published a letter from Juan Carlos to his son, King Felipe VI.

"Guided by my desire to do what is best to serve the Spanish people, its institutions and you as king, I am informing you of my … decision to leave Spain at this time," a palace statement quoted the letter as saying.

Juan Carlos, in the letter, says he made the decision against the backdrop of "public repercussions of certain episodes of my past private life".

He says he wants to ensure he doesn't make his son's role difficult, adding "my legacy and my own dignity, demand that it should be so".

He did not say which country he will be moving to. Former king says he will co-operate with corruption probe

Spanish royal family

Former King Juan Carlos, pictured with Spain's royal family in 2014, has faced a Juan Carlos's lawyer, Javier Sanchez-Junco Mans, said in an emailed statement that the former king had asked him to make clear that even though he will be outside Spain he intends to be available to cooperate with the investigation.

Spain's Prime Minister, Pedro Sánchez, said he found the developments about Juan Carlos — including investigations in Spain and Switzerland — "disturbing".

Since Spain's Supreme Court opened its probe earlier this year, Spanish media outlets have published damaging testimony from a separate Swiss investigation into millions of euros that were allegedly given to Juan Carlos by Saudi Arabia's late King Abdullah.

Juan Carlos allegedly then transferred a large amount to a former companion in what investigators are considering as a possible attempt to hide the money from authorities.

King Felipe thanked Juan Carlos for his decision, underlining "the historic importance that his father's reign represents" for democracy in Spain.

He also reaffirmed "the principles and values on which [democracy] is based according to our Constitution and legal framework".

Spain's King Felipe looks at the camera as he delivers a speech in his office.

Spanish monarchs have immunity during their reign but Juan Carlos abdicated in 2014, potentially leaving himself vulnerable to prosecution.

After media reports claimed Felipe was a beneficiary of an offshore account holding an alleged 65 million-euro ($90 million) gift from Saudi Arabia to Juan Carlos, Felipe renounced any future personal inheritance he might receive from the former king.

Felipe renounced any future personal inheritance he might receive from the former king. He also put an end to his father's palace allowance of 194,232 euros per year.

The royal house has denied that Felipe had any knowledge of his father's alleged financial irregularities.

Compiled from various news outlets