A SPECIALIST report investigating the costs of installing a new power line across the North Somerset countryside has revealed that overhead cables are the cheapest option.

The Infrastructure Planning Committee (IPC) commissioned the report to look at the cost of various options of installing a new 400,000 volt power line from Avonmouth to Hinkley Point.

The report has been put together by the Institute of Engineering and Technology and consultants Parsons Brinckerhoff.

It analyses the whole life costs of installing and maintaining new high voltage transmission circuits under the ground, under the sea and over ground.

The findings will be used by the IPC when it considers National Grid's planning application for the new line.

National Grid says the line – a large section of which will cross the North Somerset countryside – is needed to bring electricity onto its transmission network as part of the Hinkley C Connection Project.

The preferred route for the new line – which will mainly follow the route of the existing 132,000 volt line on the western fringes of Nailsea – was announced by National Grid in September.

It said the new line would be mainly over ground, but it would consider putting the cables underground "if the economies were right".

The IPC ordered the independent report after concerns that not all options for the new line had been properly investigated or costed.

The report has revealed the cost of overhead lines as the cheapest at between £2.2 million and £4.2 million a kilometre.

It said undergrounding the cables could cost between £10.2 million and £24.1 million per kilometre while underground gas insulated lines would cost between £13.1 million and £16.2 million per kilometre.

The report added that using a tunnel to underground the lines would also be an expensive option, at between £12.9 million and £23.9 million per kilometre.

The subsea option has been priced at between £13.1 million to £31.8 million per kilometre, but the cost per kilometre would reduce on longer connections.

However in the report experts warn that a decision on the technology used should not be based on cost alone.

Campaigners have been pressing for the new power line to go underground or subsea along the estuary.

Campaigners in Nailsea say the report's findings that overhead cables were the cheapest option came as "no surprise".

Nailsea Action Against Pylons group spokesman, Ian Morrell, said: "It's no surprise that overhead pylons are still considered the cheapest option, but they are fundamentally backward- looking technology and all the campaign groups are looking for more forward- looking technical solutions which take into account all the costs and environmental impact."

National Grid's major infrastructure development manager, David Mercer, said: "This report will be a valuable contribution to the public debate on the right balance between visual impact and costs that must ultimately be paid for by consumers."

National Grid has until 2013 to submit its proposals to Government and a final decision on the power line will be made by the IPC.