Only an impact study stands in the way of the Flemish authorities' deciding to route the road through the heart of the Pilkem Ridge battlefield, the site of the opening infantry campaign in the third battle of Ypres - Wipers to the British troops who fought and died there in July-August 1917.

If the authorities chose the candidate route the motorway will pass perilously close to about a dozen war cemeteries maintained by the Commonwealth war graves commission, and cut the battlefield site in two, leaving some of the tens of thousands of British troops still listed as missing in action under a carpet of concrete and Tarmac.

The authorities are due to decide early next year on the route of the bypass, which is intended to reduce traffic through several villages.

The road, connecting the area to the coast, should open in 2004 or 2005.

Piet Chielens, of the In Flanders Fields Museum in Ypres - Ieper in Flemish - said that to build the road there would be a historical mistake of epic proportions. It should be placed in a historically less significant area.

"Building over Pilkem Ridge is like building over the battlefield at Waterloo - it's as important for European history," he told the Guardian yesterday.

"From an educational and historical point of view, if you build a road straight through this battlefield you will completely and utterly destroy the traces of how the offensive went, and if you want to follow in the footsteps of the battle in future you just won't be able to do so.

"This was the biggest thing that ever happened in the war at Ieper, and the battlefield has been left almost untouched for the last 84 years."

More than 30,000 British soldiers died in the three days of Pilkem Ridge. By November, when Third Ypres ended, the British had managed to advance only five miles, to the obliterated village of Passchendaele. Historians reckon that more than 300,000 British soldiers died, along with 200,000 Germans.

"Bar the Somme, this was the British army's worst catastrophe," Mr Chielens said .

The war graves commission says it is also concerned about the roadbuilding plan, but stresses that its remit is preserving war graves not battlefields.

And, the commission says, it is not uncommon in France and Belgium for the authorities to build bypasses and main roads right beside cemeteries. Of greater concern is the fact that almost 100,000 British bodies lie concealed in the area's rich soil.

"We all know what contractors are like," a commission spokesman, Barry Murphy, said.

"If you have a lot of road works and they are up against a deadline the last thing they're going to do is call the police, stop work and let the archaeologists on site."

The cause has been taken up by a campaigning Belgian green senator, Michiel Maertens, who wants the road located elsewhere. "This plan is not on : we have to do something," he said.

"This area is virginal: it has remained untouched and the farms and roads are just as they were in 1917. It must stay like this."

A spokesman for the Flemish authorities confirmed yesterday that the Pilkem Ridge site was a candidate for the new motorway but stressed that a final decision would only be taken after a proper impact study.

There was a glimmer of hope last night, however, when sources close to the Flanders environment minister said he was willing to consider listing the battlefield as a world heritage site. But a thorough investigation into the area's historical value is needed before that can be done.

Meanwhile British officials are still checking whether plans for a third Paris airport will involve relocating first world war cemeteries.