Opponents argue it will plunge a healthy economy into recession, and deprive hundreds of thousands of Swiss citizens of their freedom to live and work across Europe.

The justice minister says that would create a situation "worse than Brexit".

Why does non-EU member Switzerland need to bother with free movement?

Switzerland decided long ago not to join the EU, but it does want access to Europe's free-trade area, and it wants to co-operate with Brussels in areas like transport, the environment, and research and education.

The price for this is to sign up to the EU's major policy "pillars" including free movement, and Schengen open borders.

The EU has consistently told the Swiss there will be no cherry-picking: leaving free movement would mean leaving those lucrative trade arrangements too.

Why now?

The proposal comes from the right-wing Swiss People's Party (SVP), and is a successor to a referendum held in 2014 (also from the People's Party) to introduce quotas on immigrants from the EU.

That passed by a whisker (50.33% said yes, 49.67% said no), obliging the Swiss government to find a deal Brussels would accept. In the end a compromise was agreed: Swiss employers had to prioritise workers permanently resident in Switzerland, in job sectors where unemployment is already high.

The SVP dismissed the deal as so weak as to be virtually meaningless, and are now back with a second demand to get out of free movement altogether.

So what are the pros and cons?

Thomas Aeschi of the SVP believes abandoning free movement will bring all sorts of advantages, from "being able to select the highest qualified immigrants" to "less land speculation, lower house prices, and lower rents".

He does not fear the loss of trade deals, arguing the only likely effect is "the Swiss will eat less French cheese and the French will eat less Swiss cheese".

Stefan Manser-Egli of the group Operation Libero, which is campaigning to keep free movement, fears the economic consequences.

Although Switzerland does trade outside Europe (it has negotiated its own bespoke deal with China), the EU is by far its biggest partner, and Switzerland's trade with Germany alone is more than its trade with China and the US combined.

Also worrying, he says, is the clause in the proposal that gives the government just one year from the vote being passed to bring it into law, with or without any new deal with Brussels.

"This means we are binding ourselves to a time-bomb," he says.

Meanwhile, half a million Swiss citizens already living and working elsewhere in Europe are worried about lost opportunities.

Carla Allenbach, who has just begun studying for a masters degree in Brussels, says: "Right now I have the same rights to work and study as EU citizens. If we say yes to this proposal, I will lose those rights."

And Franz Muheim, a professor of particle physics at Edinburgh University for almost 30 years, argues that "the future of Switzerland is at stake" if the vote is passed. He believes "the world has moved on… we are all Europeans if we live in Europe, independent of where we come from".