Swiss campaigners have moved a step closer to removing the tax privileges afforded to millionaire foreigners, after meeting the requirements for a vote on whether to abolish the controversial tax policy.

Currently wealthy residents in 21 of the 26 cantons are taxed based on the cost of living, as opposed to their income - provided that they are not in gainful employment within the country.

The generous tax regime is used by approximately 5,000 non-Swiss nationals, who the government acknowledges contribute close to 700m Swiss francs (£469.5m) in tax revenues, but has nonetheless become increasingly unpopular, with five cantons - including Zurich - having already removed the flat tax system from their jurisdiction since 2009.

Now the Alternative Left party has succeeded in securing the 100,000 signatures needed to force a national referendum on the issue and it is expected that a vote will be carried out at some point over the next two years as to whether it should be outlawed across the whole of Switzerland.

Defenders of the current setup cite the exodus of millionaires from the Swiss capital when it banned the flat tax three years ago.