Finland wants to break a deadlock on a European Union dispute over whether nuclear power and fossil fuels such as coal and gas should qualify as a sustainable investment.

With climate change abatement a priority during its six-month role as EU presidency that began July 1, Finland hopes to get member states’ backing for a plan that would allow nuclear, coal, and gas to be considered “transition” technologies that will move member states toward carbon-neutral emissions.

The dispute over nuclear power and coal is tied to a pending classification system, which the European Commission calls taxonomy, that...