An initial vote count published by SRF showed 58% of participants backed the extra pension, a 13th monthly payment per year, with just 42% against, pointing to a more emphatic victory than final polls had suggested.

The measure, which was promoted by the Swiss Trade Union Federation and left-of-centre parties, requires backing from a majority of Switzerland's 26 cantons to pass. The margin of victory meant that was likely secure, the broadcaster said.

"This step is really a huge milestone from a union perspective," Lukas Golder of polling firm gfs.bern told SRF.

The government, business lobbies and parliament, which currently leans to the right, had rejected the proposal as financially unsound. Swiss voters have in the past been cautious about backing measures viewed as risky for business.

The pension vote contrasts with referenda in recent decades in which Switzerland decisively rejected proposals that would have shortened the working week and given people more holidays.

Concern about the cost of living is widespread. Zurich, Switzerland's biggest conurbation, tied with Singapore as the world's most expensive city in a study published in November by the Economist Intelligence Unit.

Mery, a 65-year-old Zurich voter, said increased pension payments made sense.

"I'm retired now and so obviously I would like a bit more," she said, declining to give her full name. "It should allow me to give a little something to my grandchildren."

It is not clear how the pension boost, which would take effect from 2026, will be funded.

Switzerland's minimum old age and survivors (AHV) pension is 1,225 Swiss francs ($1,393) a month, and the maximum 2,450 francs. For couples it is capped at 3,675 francs.

Swiss also voted on an initiative to raise the statutory retirement age, which was comfortably defeated.