The government is facing calls by parliament to make knowledge of one of the four national languages a prerequisite for obtaining a Swiss passport.

The House of Representatives on Thursday approved a motion demanding applicants for citizenship speak German, French, Italian or Romansh fluently.

Speakers said it was key for integration into society for foreigners to be fluent in a national idiom.

The proposal, which still has to be discussed by the Senate, stops short of supporting a call by the rightwing People's Party that written skills should also be tested.

Justice Minister Eveline Widmer-Schlumpf said the government would include the latest proposal in a legal amendment next year.

In a related move, the House of Representatives backed a crackdown on marriages of convenience.

Despite opposition by the centre-left, it decided to extend from five to eight years the period in which marriages entered into for social or economic benefit can be declared invalid.

Supporters argued the current statute was too short and was often undermined by lengthy legal procedures.

The Federal Migration Office investigates about 500 suspected marriages of convenience every year.

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