That proposal appears set to fail, according to opinion polls, while the Swiss are also set to reject a planned reform of the financing of the wealthy Alpine nation's pension system slammed by unions as a "scam".

Most people cast their ballots in advance in the popular votes held every couple of months under Switzerland's direct democratic system and polling stations will only open a few hours Sunday morning.

Early results are expected by mid-afternoon.

The first proposal, entitled "For the future of our nature and our landscape", has the backing of a number of environmental protection organisations, including Pro Natura and BirdLife.

These warn that Switzerland's biodiversity "has declined".

"Switzerland has one of the highest levels of threatened species and environments among the European countries in the OECD (Organization for Economic Cooperation)", Sarah Pearson Perret, a Pro Natura director, told AFP, citing a report by the European Environmental Agency.

Published in 2020, that study shows that the percentage of protected zones compared to the national territory is lower in Switzerland than anywhere else in Europe.

- 'Alarming' -

Switzerland's federal government currently invests around 600 million Swiss francs ($700 million) each year towards the preservation of biodiversity.

But the organisations behind Sunday's referendum say that is not enough.

While they have not said exactly how much more should be spent, they are asking voters to approve boosting the biodiversity budget and to expand the number of protected areas.

A landmark biodiversity agreement reached in Montreal in 2022 called for at least 30 percent of the planet's lands and oceans to be protected by 2030.

But last year, Switzerland's Federal Office for the Environment (FOEN) said just 13.4 percent of the Alpine country's territory was dedicated to biodiversity conservation.

The FOEN acknowledges that Swiss biodiversity protection is insufficient, with half of all natural environments and a third of natural spaces threatened.

Aquatic environments and marshes are among the most threatened.

The organisations backing the referendum deem the situation "alarming", stressing that "the mass extinction of species directly affects us humans".

Both the government and parliament have come out against the proposal, insisting Switzerland is already doing enough and warning that the measures would severely impact the economy, agriculture, construction and energy production.

- 'Scam' -

Swiss voters also appeared set Sunday to reject a government-backed reform of pension financing.

Today's Swiss pensions are split into three so-called pillars: the basic state pension, a compulsory pension fund into which employers and employees must pay and voluntary top-ups into private funds and investments.

According to the government, low financial market returns and rising life expectancy have left the second pillar underfinanced.

Bern's proposed reform would require employers and workers to raise their pensions contributions into the obligatory occupational funds.

But the unions who demanded the reform be put to a referendum charge that it would force people to contribute more even as they would see their pension payments shrink.

The Swiss Trade Union Federation, an umbrella group of 20 unions, has slammed the reform as a "scam".