GENEVA (AFP) - Swiss voters were split down the middle on Sunday on whether to curb immigration by European Union citizens, a polling agency said, in a referendum seen as a crunch test of the neutral country's ties with the 28-member bloc.

Based on a combination of partial results and estimates, the gfs.bern public opinion institute said the result appeared too close to call. It estimated that 50 per cent of voters had backed either side in the Stop Mass Immigration referendum, with a margin of error of 3 per cent.

The measure was crafted by the powerful right-wing populist Swiss People's Party, which is hawkish about the sovereignty of this non-EU member state and says the country is being swamped.

The vote was also being watched closely by eurosceptics within the EU who want to rein in immigration among its member states, notably from eastern to western Europe.

If passed, the measure would bind the Swiss government to renegotiate within three years a deal with Brussels that since 2007 has given most EU citizens free access to the country's labour market.

The populists say that with 80,000 EU citizens arriving per year - rather than the 8,000 predicted before the rules were liberalised - it is time for the nation of eight million people to rein things in.

Their opponents, including the government and lobby groups from across the economy, say immigrants are needed to drive Swiss business and industry, and that ripping up the free movement deal would mean the demise of a handful of related economic accords with the EU.

The referendum needs more than 50 per cent to pass, as well as a majority of Switzerland's 26 cantons, which are the equivalent of US states.

Switzerland is ringed by EU member countries and does the bulk of its trade with the bloc.

The labour market accord is part of a raft of deals signed with the EU in 1999 after five years of talks, approved by Swiss voters in 2000 and phased in.

Its supporters say Switzerland's credibility as a negotiating partner would be ruined if it ripped up the labour rules.

They also argue that restricting the hiring of EU citizens would act as a brake on the wealthy Swiss economy, which enjoys virtually full employment but has an ageing population.

Critics of the migration control plan underline that the treaty with the EU allows Switzerland to reimpose temporary quotas - something it has deployed to control numbers of workers from the EU's ex-communist member states.

But the quota clause expires this year.

Brussels warns that Switzerland cannot pick and choose from the binding package of deals negotiated painstakingly in the 1990s, seen as a way for the country to enjoy the benefits of access to the EU market without membership.

"If the 'Yes' camp wins, there'll be total chaos and a huge period of uncertainty in relations with the EU," University of Geneva political scientist Pascal Sciarini told AFP.

Lawmaker Hans Gruender said a "Yes" vote could also have huge domestic impact, saying a government reshuffle could be on the cards.

Switzerland is governed by a cabinet including the Swiss People's Party, but the other four parties opposed the measure, and centrist Gruender told the newspaper Schweiz am Sonntag that the right-wingers should be forced to shoulder the responsiblity for a crisis.

The Swiss political system gives the people the last word on a huge range of issues and referendums are common, and the populists mustered over 135,000 signatures to force a vote.

They argue that EU citizens undercut Swiss workers, and that overpopulation has driven up rents, stretched the health and education systems, overloaded the road and rail networks, and eaten into the landscape due to housing construction.

In a nod towards such concerns, the government recently adopted measures making it harder for newly arrived EU citizens to apply for Swiss social security.

Immigration and national identity are traditional political themes in a country with a long history of drawing foreign workers and some of Europe's toughest rules for obtaining citizenship.

But over recent years, the proportion of foreigners has risen from around one-fifth of the population to roughly a quarter.

The majority of recent immigrants are from neighbouring Germany, Italy and France, as well as Portugal.