"The government must put together a list of all real estate acquired by Franco from the state during his rule, which his heirs now control, and return it to government ownership," the source quoted a statement by deputies of the United Left coalition as saying.
Parliamentarians from the northwestern region of Galicia, where much of the property in question is located, added their support to the coalition's proposal.
The initiative was launched when Franco's daughter Carmen and her seven children refused a request by Galicia's local administration to permit visitors into the dictator's summer residence in Pazo de Meiras twice a month.
The palace is considered a masterpiece of 19th century architecture and is part of the national heritage.
According to Spanish law, owners of historically or architecturally notable buildings are required to permit public excursions on their property.
The Madrid newspaper Pais has reported that the fortress in question could be worth as much as 600 million euros, although it has never been formally appraised.
In addition to the palace at Pazo de Meiras, Franco acquired over the years the ancient Canto del Pico palace, near Madrid, a residence in the town of La Cornuda, in Galicia, and a number of lesser homes.
"The dictator's heirs control tens of other, smaller properties ... which permit them to live in luxury, even though not one member of the family has ever worked," Pais said. The family faces a 60,000-euro fine for their defiance.
No fewer than 50 real estate firms and stock companies manage the properties, the paper said.
In addition to real estate, Pais reported, Franco's heirs own other gifts presented to the late dictator by the Spanish government during his nearly 40-year rule, including artwork and jewelry.
Franco seized power in Spain in 1939 following a bloody fascist uprising and ensuing civil war against an elected left-wing government that left up to half a million people dead.
He established a durable dictatorship, surviving the war that destroyed his fascist Italian and German allies, and retained uncontested power until his death in 1975.
