RARE blood-sucking fish are set to rear their ugly heads again, weeks after causing a £197,000 bridge conservation scheme to be halted.

Natural England has warned protected river and sea lampreys, which are considered key to maintaining river ecosystems, could be damaged by a proposed hydro-electric plant on the River Swale, at Topcliffe, near Thirsk, North Yorkshire.

The government’s adviser on the natural environment has recommended Harrogate Borough Council reject a plan by Richmond-based UK Hydro Ltd to generate enough electricity to power 539,000 100W lightbulbs for an hour, stating it could damage a designated Site of Special Scientific Interest more than 40 miles away.

Earlier this month, North Yorkshire County Council was forced to postpone work five miles downstream, at Thornton Bridge, at Helperby, near Thirsk, after the Environment Agency issued an alert that lampreys breeding there could be upset.

Lampreys, which resemble eels and can grow up to 90cm long, are considered important to protect as they are among the few survivors of the jawless stage in vertebrate evolution.

Hydro UK Ltd's renewable energy scheme would involve excavating land beside the river to create a channel, through which water will be diverted to power an Archimedean screw.

Natural England has called for the borough council to assess the implications of the proposal, following a report commissioned by UK Hydro Ltd stating the scheme would "have little to no effect on lamprey spawning and migration at the site".

It states: "The turbine and fish pass will act as a safe additional route for downstream migrants that will also be able to continue to pass over the weir.

"The fish, eel and lamprey passage facilities will vastly improve habitat connectivity at the site and will provide better access to over 30km of upstream habitat, and as a result will significantly contribute to meeting Water Framework Directive targets."

Natural England has questioned numerous aspects of the report and said a suggestion that the Archimedean screws could be fitted with rubber buffers could lead to lamprey suffering fatal injuries.

North Yorkshire-based Bellflask Ecological Survey Team, which has monitored the river's lamprey population for 15 years, said since the installation of a hydropower turbine on the River Ure, at Mickley, near Masham, no river lamprey had been recorded upstream.

A spokesman for the borough council said: "We are currently working towards trying to resolve the outstanding issues with the application and hope that matters will be drawn to a conclusion soon so that the application can be presented to the planning committee."