Swiss voters have approved a measure to make it easier for third-generation immigrants to become citizens, in a defeat for right-wing nationalists who carried out an anti-Islam campaign in the run-up to the vote.

According to final official results, the "Yes" camp claimed 60 percent support and a victory in 19 of Switzerland's 26 cantons, meeting the two criteria needed for a win.

The government, as well as most politicians and political parties, supported the proposal that would allow the grandchildren of immigrants to skip several steps in the lengthy process.

However, the far-right nationalist Swiss People's Party (SVP), which controls the highest number of seats in the National Council, campaigned for "No" putting the issues of Islam and national identity at the centre of the debate.

Reacting to the defeat, Jean-Luc Addor, SVP member of parliament, said his side was "alone against everyone in this campaign".

"The problem of Islam, I'm afraid, it will catch up with us in a few years," he told RTS television.

Nearly 60 percent of the eligible third-generation immigrants are Italians, followed by those with origins in the Balkans and Turkish nationals.

Poster controversy

As a part of the "No" campaign, a widely distributed poster showed a woman with shadowed eyes staring out from a black face veil with a tagline urging voters to reject "uncontrolled citizenship".

The SVP is not officially responsible for the poster.

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It was commissioned by the Committee Against Facilitated Citizenship, which has several SVP members, including some in leadership positions.

The co-chairperson of that committee and an SVP politician, Jean-Luc Addor, urged people to vote "No" on grounds that in the coming years most third-generation immigrants will not be of European origin.

"In one or two generations, who will these third-generation foreigners be?" he wrote in an opinion piece on the SVP website.

"They will be born of the Arab Spring, they will be from sub-Saharan Africa, the Horn of Africa, Syria or Afghanistan," said Addor, who has defended the niqab poster.

Critics of the inflammatory campaign image have denounced it as a brazen appeal to those worried about more Muslims becoming Swiss.

The SVP in 2009 successfully persuaded Swiss voters to approve a ban on new mosque minaret construction, while religiously charged messages have been a part of multiple referendums on immigration since.