A federal court ruling has thrown out the preliminary approvals for a series of new nuclear power reactors in Ontario.

Ruling in a case brought by environmental groups, Justice James Russell says the environmental assessment for the proposed expansion of the Darlington nuclear plant fell short.

Russell says the assessment should have examined the environmental effects of radioactive fuel waste, a Fukushima-type accident and hazardous emissions.

As a result of the decision, the whole project is stalled until a panel can redo the assessment.

Ontario Power Generation's plan to expand Darlington has been in the works since 2006 and would have seen up to four new reactors built.

Environmentalists welcomed the ruling.