Norfolk villagers have triumphed today in their High Court fight to block plans for a lorry park and industrial units they see as a threat to rare flora and fauna in a nearby river.

For more than two years, residents of Great Ryburgh, near Fakenham, have fought to scotch proposals by village maltster, Crisp Maltings Group Ltd, to extend its premises with a lorry park, silos and a fuel storage unit on a greenfield site.

Villagers say the development would risk polluting the nearby River Wensum, which is home to rare white-clawed crayfish, Desmoulin's wort snail, brook lamprey, bullhead and ranunculus plants that flourish in its chalk-rich stream.

North Norfolk District Council granted Crisp Maltings planning permission in September 2011. But a top judge today sent planners back to the drawing board when he overturned that decision, pinpointing a fundamental 'inconsistency' in the council's approach.

Judge James Dingemans QC said it was possible for the council to rationally conclude, after taking advice from Natural England and others, that it could grant consent without carrying out a full environmental impact assessment or habitats appropriate assessment.

However, the council's development control committee could only do that if it was convinced that there was 'no relevant risk' of the river suffering pollution.

By imposing as a condition of the planning consent a regime for regularly testing the river for pollutants, the judge said the committee implied that a 'residual risk' existed. The committee could not 'rationally adopt both positions at once', he ruled.

The judge's decision means the council must consider Crisp Maltings' application afresh and, if it identifies any pollution risk at all, it must carry out full environmental and habitat assessments before it can lawfully grant planning permission.

Crisp Maltings, which has been based in the village since 1890, first applied for permission to build the lorry park, with 25 HGV and 21 car parking spaces, two silos capable of holding 6,000 tonnes of barley, fuel storage and other facilities, in October, 2009.

Despite the proximity of the river, a site of special scientific interest, the council decided that the plans would not create any environmental risks requiring special assessment.

Campaigners, however, insist planners got it wrong and that polluted ground water could leak into the river through a system of ditches if there is 'extreme rainfall' or the company's counter-measures malfunction.

The judge said Crisp Maltings had carried out its own detailed environmental and flood risk assessments in a bid to allay those fears.

The council took advice from Natural England, the Environment Agency and the Norfolk Rivers Internal Drainage Board before deciding that a full environmental impact assessment was unnecessary and granting planning consent subject to tight restrictions.

Council officers took the view that mitigation measures proposed by Crisp Maltings were 'tried and tested' and that there was 'no relevant risk' to the river waters.

Ruling on the case, Judge Dingemans said there was evidence that 'there might have been a residual risk' and that was backed up by the committee's insistence on a strict water quality testing regime to ensure the river was not being contaminated.

He told the court: 'It does not seem to me that the council could, rationally, adopt both positions at once...I do not consider that it is open for me to consider that this inconsistency was simply a function of local democracy at work and that it could be ignored.'

Overturning the planning permission, he concluded: 'The committee will have to consider whether it considers that there is no relevant risk of pollutants entering the river...if there is such a risk, the committee will have to require an habitats appropriate assessment and an environmental impact assessment to be obtained'.

Crisp Maltings argues that the proposed extension to its site would save almost 5,000 HGV miles-a-year, as it would remove the need to transport vast amounts of barley to another plant, although more car journeys would result.