The Swiss will once again head to the polls this weekend to decide whether to dramatically slash immigration numbers, this time in the name of saving the environment in a proposal opponents have labelled xenophobic.

Initiatives aimed at reining in the number of immigrants, asylum seekers and refugees to the wealthy Alpine nation have been abundant in recent years.

Most recently, voters in February narrowly approved reinstating quotas for immigration from the neighbouring European Union, throwing non-member Switzerland's relations with the bloc into turmoil.

But while saving Swiss jobs and culture are often cited as reasons to tighten the borders, Sunday's vote marks the first justifying its demand to cut immigrant numbers with their impact on the country's environment.

The so-called Ecopop initiative maintains the current influx of foreigners is swelling the country's population and cities and shrinking its idyllic landscapes and green spaces.

"Population growth is at historic levels in Switzerland," said Anita Messere, part of the Ecopop committee behind the initiative.

- 'Being covered in concrete' -

"This is really harming the environment," she told AFP, arguing that the inhabitable plains of the mountainous country were being covered in concrete at a rate of more than one metre (three feet) per second.

Foreign nationals already make up nearly a quarter of Switzerland's some eight million inhabitants, according to official statistics.

And immigration is adding 1.1-1.4 percent annually to the Swiss population, according to Ecopop, which wants the country to push that percentage down to the European average of just 0.2 percent.

Once balanced against the some 93,000 people who emigrate from Switzerland each year, immigration would at that rate add around 16,000 more people to the population annually, allowing it to grow to 8.5 million by 2050.

But if nothing is done, Switzerland will count as many as 12 million people by 2050, according to the Ecopop campaign.

To help rein in over-population beyond Switzerland's borders, it also calls for 10 percent of the country's development aid budget to be dedicated to family planning initiatives abroad.

The government, all the political parties, employers and unions have rejected the initiative, slammed by some as xenophobic, and a poll published last week hinted 56 percent of voters will do the same.

Observers point out however that surveys often underestimate the backing for populist initiatives.

"There could be a surprise on November 30," Messere said.

Christian Luescher, a parliamentarian for the Liberal Party and co-chair of the committee opposing Ecopop acknowledged support for the initiative may be underestimated in the polls.

"You can understand why people might hesitate to say they plan to vote for Ecopop," he told AFP.

But he said he remained confident the Swiss would vote down "this absolutely absurd initiative," which by depriving the heavily-immigration dependent Swiss economy of much of its workforce would "impoverish our country."

Many voters meanwhile appeared conflicted.

"Ecopop raises a real, fundamental question," said Gabriella Queix, who works in television.

"But the solutions it proposes are no good," she told AFP, stressing that "you can't stop people from moving, and it's not by being racist and xenophobic that you're going to preserve your little plot of land."

Student Roger Gulke however described the proposal as "a true ecological initiative," although he acknowledged he was unsure it was applicable.

- Scrap tax-breaks for foreign millionaires? -

As part of Switzerland's direct democratic system, the Swiss will also vote Sunday on whether to end a preferential tax system for wealthy foreigners residing in the country.

The initiative, backed by left-leaning political parties and unions, wants to axe a system that allows rich resident foreigners who do not work in the country to pay a lump sum tax based on their spending rather than income.

Switzerland counts 5,729 millionaires and billionaires with foreign passports, who together pay around one billion Swiss francs ($1.04 billion, 830 million euros) in taxes annually.

That is a far cry from what they would have paid had they been levied the same percentages as average Swiss tax payers, the initiators say.

But backers of the system point out that these wealthy foreigners still contribute heavily to Swiss tax coffers, in addition to the money they inject directly into the local economy.

"We need these people," Luescher insisted, warning that if they were taxed more many would leave, and would be welcomed elsewhere "with open arms."

Last week's poll indicated most Swiss agree, although 42 percent said they would vote to scrap the system.

International relations student Cecile Genoud is one of them.

"I can't see why we should give preferential treatment to the part of the population that is already the most well-off," she told AFP.