Spain's former king Juan Carlos, at the centre of an alleged $100-million corruption scandal, has reportedly fled to Portugal after a shock announcement he was going into exile.

The 82-year-old revealed Monday that he had decided to leave Spain to help his son, the current King Felipe VI, "exercise his responsibilities".

The letter, published by the royal palace, did not mention where he would go, nor when exactly he would leave.

Spanish media went into overdrive Tuesday about where Juan Carlos had fled as the palace refused to reveal further information.

Online newspaper El Confidencial said the former head of state was in Portugal, where he spent part of his youth, at the town of Azeitao, 30 kilometres (18 miles) south of the capital Lisbon.

He was "staying with the Brito e Cunha-Espirito Santo family that he has maintained friendly relations with" since his own family's exile during the Franco dictatorship.

"Until further notice, this will be his residence after he ruled out other more distant destinations such as the Dominican Republic or New Zealand," said El Confidencial.

Daily newspaper ABC had earlier reported that Juan Carlos left Spain on Sunday and flew to the Dominican Republic via Portugal.

Juan Carlos (L) ascended the throne in 1975 on the death of the fascist dictator Francisco Franco and ruled for 38 years / © AFP/File

Dominican television station Canal de Noticias even said the country was "carrying out preparations for his arrival".

However the Republic soon denied that the ex-king had entered the country. The foreign ministry told AFP it had "no information" about Juan Carlos eventually spending time there.

Im Madrid, a royal palace spokesman refused to reveal his whereabouts saying: "The only information we have is the information which was published on the website of the royal palace yesterday."

Socialist Prime Minister Pedro Sanchez said he did not know where Juan Carlos had gone but suggested King Felipe had pressured his father to go abroad.

"The government and I completely respect the decision of the royal palace to distance itself from the questionable and reprehensible conduct" of a member of the royal family, Sanchez told a press conference.

Former queen Sofia, who has lived separately from the ex-king for many years, is still in Spain, a source close the palace told AFP.

- Saudi funds -

Juan Carlos has been under a cloud since reports surfaced that he allegedly received funds from Saudi Arabia and probes are now under way in both Switzerland and Spain.

Spain's Supreme Court announced in June an investigation to determine his legal responsibility -- but only for acts committed after his abdication in 2014, because of the immunity he holds.

Juan Carlos I / © AFP

The suspicions centre on $100 million (85 million euros) allegedly paid secretly into a Swiss bank account in 2008.

An inquiry opened in Spain in September 2018 following the publication of records attributed to German businesswoman Corinna Larsen, a former mistress of Juan Carlos.

She claimed he had received a commission when a consortium of Spanish companies were awarded a high-speed railway contract to link the Muslim holy cities of Mecca and Medina in Saudi Arabia.

Larsen told Swiss investigators he had transferred to her nearly 65 million euros in the Bahamas, "not to get rid of the money", but "out of gratitude and out of love", according to El Pais daily.

- 'Distanced itself' -

Swiss media reported last March that Juan Carlos was paid $100 million into a Panamanian foundation's Swiss bank account by late Saudi king Abdullah in 2008. Britain's Daily Telegraph reported that Felipe VI was also a beneficiary of the foundation.

In response, the king withdrew from his father an annual royal stipend of nearly 200,000 euros and renounced his inheritance.

The former king's lawyer said his client was not trying to escape justice by going into exile and would remain available to prosecutors / © AFP/File

"The media have done their job in denouncing these alleged irregular practices, judges and prosecutors have taken action...and the Royal House has distanced itself from these alleged behaviours, and I stress alleged, which could be irregular," Sanchez said.

Juan Carlos ascended the throne in 1975 on the death of the fascist dictator Francisco Franco and ruled for 38 years before abdicating in favour of his son Felipe VI in June 2014.

He was a popular figure for decades, playing a key role in the democratic transition from the Franco dictatorship which ruled Spain from 1939-1975.