The court did not provide further details as the case is sealed and preliminary, only saying it followed a complaint raised by anti-graft campaign group Manos Limpias - Clean Hands - whose leader has links to the far-right.

Manos Limpias said Gomez used her influence as the wife of the prime minister to allegedly secure sponsors for a university master's degree course that she ran.

Sanchez told lawmakers in congress after the news was confirmed that he had faith in the justice system.

"On a day like today, and despite the news I have heard, despite everything, I still believe in justice in my country," he said.

Gomez could not immediately be contacted for comment.

Manos Limpias uses a unique Spanish legal instrument known as the "people's accusation" to bring criminal complaints against third parties.

Judges can choose whether or not to act on such complaints, and not every accusation has led to a judicial investigation. However some have: Clean Hands' allegations against Princess Cristina, King Juan Carlos's younger daughter, led to her prosecution over claims of being an accessory to tax fraud. Her husband was convicted for fraud and tax evasion but she was acquitted.

Earlier this month, a complaint against Sanchez for the law amnestying Catalan politicians over the region's independence drive was not admitted by the Supreme Court.

"It's a false accusation by a far-right organisation to slander and defame the government premiership," vice-prime minister Maria Jesus Montero told reporters, while the main opposition People's Party, asked Sanchez to explain himself.

In Spain, politicians of all stripes routinely accuse each other along with family members and associates of corruption and there have been some high-profile scandals that resulted in convictions.

Pedro Sanchez came to power in 2018 when the previous conservative government lost a confidence vote in the wake of a corruption scandal.