Spanish Prime Minister Mariano Rajoy cannot discuss with Catalan authorities what could make him agree to a referendum on the region’s secession because the law simply doesn’t allow it, AP reported.

Officials in Catalonia and the northeastern region’s capital, Barcelona, asked in a letter released on Friday for fresh dialogue on holding the vote with the government’s permission.

“The Prime Minister can’t make something illegal into something legal,” said Inigo Mendez de Vigo, who is Minister of Culture and the spokesman for Rajoy’s cabinet, when asked about the letter.

Separatist politicians have vowed to hold the vote on October 1 despite a ban by the country’s courts.