Switzerland began voting Sunday on whether to make it easier for third generation immigrants to become citizens, after a campaign tainted by anti-Muslim messages and charges of religious prejudice.

This Sunday is one of four in the year reserved for direct voting on subjects affecting federal as well as local laws and institutions.

Voting ends at noon and the initial results are expected in the afternoon. Most people have already sent their votes by mail.

The government as well as most lawmakers and political parties support the proposal that would allow the grandchildren of immigrants to skip several steps in the lengthy process of securing a Swiss passport.

But the outcome of the referendum, the latest in Switzerland's direct democracy system, has been clouded by the far right nationalist Swiss People's Party (SVP), which put issues of Islam and national identity at the centre of the debate.

According to a migration department study, less than 25,000 people in the country of about eight million currently qualify as third generation immigrants, meaning they have at least one grandparent who was born here or acquired Swiss residency.

Nearly 60 percent of that group are Italians, followed by those with origins in the Balkans and Turkish nationals.

Debate on the proposal had nothing to do with religion at the outset, said Sophie Guignard of the Institute of Political Science at the University of Bern.

It was the SVP, a party repeatedly accused of demonising Islam, that focused on the risks of more Muslims becoming citizens and the possible "loss of Swiss values", Guignard told AFP.