"The socialist party (PSOE) will ask the government if it knew of the inheritance received by King Juan Carlos and if it can tell us if this inheritance was declared to tax officials," a spokeswoman for the Socialist Party group in parliament told new agency AFP.
The Infante Juan of Spain, Count of Barcelona left his family 1.1 billion pesetas — the equivalent of around €6.6 million ($8.5 million) — to his family in three bank accounts in Switzerland, where they had lived for several years, the newspaper said.
King Juan Carlos received 375 million pesetas, or around €2.2 million.
Several smaller parties have already presented parliament with written questions over the affair after centre-right newspaper El Mundo reported on Sunday that King Juan Carlos received an inheritance when his father Juan died on April 1, 1993.
Officials from Spain's ruling conservative Popular Party have indicated it was not in favour of looking into the inheritance with Popular Party lawmaker Rafael Hernando telling reporters Monday the issued "belongs to the past".
The royal palace, which at the end of 2011 released a breakdown of the royal family's finances for the first time as part of the king's commitment to making his household's accounts transparent, said Tuesday it was looking into the report.
"We are searching for information and once we have it we will release it," a spokesman for the royal palace told AFP.
The question of the inheritance comes at a delicate time for the royal family, which finds itself at the heart of several scandals, including a probe into allegations that the king's son-in-law, Inaki Urdangarin, embezzled million of euros in public money.
The corruption probe, and an elephant hunting trip which the king took on Botswana last year, have thrown the spotlight on the royal family's deluxe lifestyle and opaque fortune as Spain grapples with a record unemployment rate of 26 percent.
General support for having a monarchy in Spain fell to a historic low of 54 percent, according to a poll published in January in El Mundo.