Silvio Berlusconi, Italy’s prime minister, and his centre-right coalition on Wednesday redrew the lines of battle for next April’s general election by passing changes to the voting system that his opponents denounced as a scandal and a threat to democracy.

The changes will return Italy to full proportional representation, a system abolished in a 1993 referendum after voters blamed it for incompetent, short-lived governments in thrall to self-interested, corrupt political parties.