Three small opposition parties are calling for a boycott of a government-sponsored referendum seeking support for opposing any future European Union plans to resettle refugees among member countries.

Officials from the Modern Hungary Movement, Dialogue for Hungary and Together said Thursday that a billboard campaign supporting the boycott will be launched Sept 15. The parties hope to keep turnout below the 50 percent threshold needed for the vote to be valid.

The government says the referendum to be held Oct. 2 is an opportunity to reject EU migration policies and defend national sovereignty. Prime Minister Viktor Orban has described migrants as "poison" and Hungary built fences on its southern borders last year to stop the migrant flow.

Polls show a majority of Hungarians reject taking in migrants and refugees.