The socialist prime minister of Spain between 1982 and 1996 waded into the political row, just a month before Catalonia goes to the polls to urge the northeastern region not to turn away from Spain.
In an opinion article published in Spanish newspaper El Pais on Sunday, the 73-year-old ex-leader of the socialist PSOE party argued that regional president Artur Mas is leading Catalonia "in a strange and ridiculous front rejecting and breaking the law".
"I have believed, and go on believing that we are much better together than opposed: recognizing diversity as a shared asset and not as a cause for rupture between us," he wrote in an open letter entitled 'To the Catalans'.
"For me, Spain would no longer be Spain without Catalonia and neither would Catalonia be what it is separated and isolated," he said.
He argued that the consequences of "breaking away" would be to fracture Catalan society, isolating those who "do not have a Catalan pedigree".
Regional President Artur Mas has called elections for September 27th. Photo: AFP
Not only would it mean breaking away from Spain and a "statute that guarantees them self-government and the secular coexistence that we share in this public space," but it would also mean "breaking away from Europe" and from Ibero-America, and the connection with a language shared by 500 million people.
"How can they want to take the Catalan people into isolation, into a kind of 21st-century version of what Albania once was?" he asked.
"What it most resembles is the German and Italian ventures of the 1930s. But it is hard for us to express it in these terms out of respect for the tradition of coexistence in Catalonia," he said.
He also warned that Mas was tricking Catalans into believing that they would be in a stronger bargaining position if the majority vote for independence from Spain.
It’s "the same mistake that former prime minister Alexis Tsipras made in Greece, but outside of the law and with more serious results," because in the end Tsipras accepted much worse conditions than those rejected in the referendum.
"By breaking the law, they will not manage to seat anyone round a negotiating table who has the duty to respect and uphold it," González insisted. "We would all be risking what has already been achieved, and the possibility of moving forward through dialogue and reforms."
Independence supporters hold up Catalan national flags. Photo: AFP
But the former leader did also use the piece to chastise the position of the conservative government of Mariano Rajoy. "I do not agree with the intransigence of the national government, which is closed to dialogue and reform, nor with its unnecessary appeals to the Constitutional Court," he said.
The missive from González has been likened to the role played by former British prime minister Gordon Brown who called for Scotland to reject independence in a referendum held in September 2014.
The September 27th regional elections in Catalonia are being treated as a plebiscite on independence after attempts to hold their own Scottish style referendum were blocked by the Spanish government.
Called "Junts pel Si" ("Together for Yes") it groups the ruling conservative CDC party, the left-wing ERC party and civil society organisations which have staged massive pro-independence demonstrations in recent years.
But surveys show that for the first time since 2011, backing for Catalonia to remain a part of Spain surpasses support for independence.