Spain’s socialist government has claimed ownership of Francisco Franco’s summer palace in Galicia in its latest clash with the former dictator’s descendants, who have opposed a plan to exhume his remains from a state mausoleum outside Madrid.

The decision to move Franco from his tomb in the Valley of the Fallen, which is carved into a mountainside to the north of the capital and seen by many Spaniards as a monument to fascism, was suspended by the country’s supreme court in June pending appeals by his family.

Public opinion remains divided over the 1936 to 1939 Spanish civil war, which tore apart families and communities, and the legacy of the ensuing far-right dictatorship that ended with Gen Franco’s death in 1975.

More than half a million people died during the civil war and an estimated 150,000 were killed by Franco’s regime, while 450,000 were forced to leave Spain, historians estimate.

Spain’s justice ministry said in a statement on Thursday it had presented a claim to a local court in the north-west region of Galicia, arguing that the sale of the Pazo de Meirás palace to Franco was fraudulent.

“They [the government] already threatened us that there would be retaliation if we did not back down [over the Valley of the Fallen] and this is part of the strategy of retaliation,” said Francis Franco, one of Franco’s grandsons.

Spain’s acting deputy prime minister, Carmen Calvo, said the government had a “solid argument, documents and legal position to defend public ownership” of the palace, which is advertised for sale online at more than €5m (£4.5m).

The Galician palace, built between 1893 and 1907, was owned by the descendants of the writer Emilia Pardo Bazán and sold to a pro-Franco organisation in 1938 during the civil war.

The justice ministry said the funds used to buy the palace were raised by forced donations from local people. It alleges the building was then sold to Franco in 1941, in what it says was a fraudulent bill of sale.