If voters say Yes, the Alpine country's entire set of EU relations could be suspended under a guillotine clause which can be triggered if any part of the EU-Swiss agreement is violated.

The No-camp is currently polling at 50 percent, while proponents of the move have 43 percent.

The referendum target refugees, and economic migrants from poorer EU member states, but will also hit, for instance, well-heeled Germans who reside in Switzerland to pay less tax.

Some 300,000 Germans currently live in Swizterland, which has a total population of 8 million.

"How many Germans can Switzerland take?" the Swiss tabloid, Blick, asked already last year.

If the Yes vote succeeds, all newcomers will fall under strictly capped "contingents.”

The details would have to be negotiated with the EU in a process that is supposed to take no more than three years.

In the worst case scenario, Switzerland's exclusion from the single market could harm its economy, as 55 percent of its exports go to EU countries.

But the Swiss People's Party is saying that the UK may exit the EU in 2017, creating new conditions in any EU-Swiss talks.