VIENNA (Reuters) - Austria's environment minister, Leonore Gewessler of the Greens, defied her conservative coalition partners on Sunday by pledging to cast Austria's vote in favour of adopting a European nature restoration law, potentially tipping the balance in Brussels.

European Union countries' environment ministers will discuss the bloc's flagship policy to restore damaged nature at a meeting in Luxembourg on Monday and potentially hold a final vote on whether to enact it.

The law would be among the EU's biggest environmental policies, requiring member states to introduce measures restoring nature on a fifth of their land and sea by 2030.

"The time for decisiveness has come. I will vote in favour of the EU Nature Restoration Law on Monday," she told a news conference called at short notice.

EU countries had planned to approve the policy in March but called off the vote after Hungary unexpectedly withdrew its support, wiping out the slim majority in favour.

Austria's change of position would give the policy enough support to become law if no other countries switch.

Hungary, Italy, the Netherlands, Sweden, Belgium, Finland and Poland have previously said they will not support the policy but without Austria they would be one country short of being able to block it.

"This law is on a knife-edge. A majority at the European level is in no way certain," Gewessler said, adding that some countries were hesitating to support it.

Gewessler's announcement angered Chancellor Karl Nehammer's conservative People's Party (OVP), which opposes the law. It controls the Agriculture Ministry and says that since that ministry is partly responsible for this issue, Gewessler needs its backing.

The OVP minister for EU and constitutional affairs, Karoline Edtstadler, said that if Gewessler voted in favour without the Agriculture Ministry's approval it would be unconstitutional.

"That must and will have legal consequences," Edtstadler said on X.

(Reporting by Francois Murphy; Additional reporting by Kate Abnett in Brussels; Editing by Peter Graff)