The initiative said on Thursday it planned to collected the required 2,500 signatures to initiate the referendum procedure by Tuesday.

If the NGOs manage to collect the 40,000 signatures required to call the referendum, the vote could be called in early October, Vili Kovačič of the initiative explained.

The move comes after the parliament passed on Wednesday the law providing state guarantees for EUR 440m loan from the European Investment Bank.

The "Let's Stop TEŠ6" initiative believes the project poses a serious environmental threat and fails to properly address Slovenia's energy needs.

The initiative includes NGOs Greenpeace Slovenija, Focus, Ekologi brez meja (Ecologists without Borders), Umanotera, the civil initiative Davkoplačevalci se ne damo (Taxpayers Won't Give Up), the legal and information centre for NGOs, the Institute for Sustainable Development, the Movement for Sustainable Development TRS, non-parliamentary parties Zares and Youth Party (SMS Greens) and MEPs Ivo Vajgl and Lojze Peterle.