Work on a 50-mile line of pylons through the Vale of York is set to start within weeks as protesters prepare petrol crisis-style blockades of country lanes.

The National Grid has revealed it is on the verge of taking on a contractor to carry out construction work on a 400,000-volt line of pylons from Teesside to Shipton, near York, and says work will start later in the autumn.

Stewart Grant, spokesman for the National Grid, said: "Work on the section of underground cables has already started and we are likely to appoint a contractor to install the overhead lines in the next few weeks, with work starting in autumn."

But members of protest group Revolt are sceptical of the claims, saying there are still landowners who have not given their permission for lines to be erected on their land.

Mike Barr, a group spokesman, said: "National Grid have been saying they're going to start work immediately for years, but they can't build the line if they haven't surveyed the land or got permission from the land owner."

But if construction does begin, Mr Barr said protesters would make their presence known.

"We do have plans for peaceful protest," he said.

"What the lorry drivers did in the petrol protest is exactly the strategy we're after. We've worked out it takes three lorries to block a motorway and it takes one man and his dog to block off a country lane."

Meanwhile, Revolt has designed a T-shirt showing an upturned pylon in the shape of the Olympic torch to mark ten years of peaceful protest.

Professor Denis Henshaw, of the University of Bristol, has recently made new links between the electromagnetic fields produced by power lines and air pollution.

He said: "High voltage power lines ionise the air...ions attach themselves to pollutant particles in the air like vehicle exhausts and crop sprays so that when the particles are inhaled they have a greater probability of being absorbed by the body."

But Stewart Grant said: "Alleged health risks have been investigated throughout the world over the last 20 years and the weight of evidence is against there being health effects."

Revolt is holding its AGM at Thirsk Town Hall on October 5, at 7.30pm.