THE GOVERNMENT'S controversial £4 billion plan to build another road bridge across the Firth of Forth is about to run into an unexpected hitch: a historic toilet.

A first world war latrine at Port Edgar in South Queensferry, along with a cell block, an air-raid shelter and naval barracks, have all been put under legal protection by the government's guardian of ancient monuments, Historic Scotland. But the buildings are directly in the way of the proposed new bridge.

The Sunday Herald can also reveal that another government agency, Transport Scotland, secretly tried to prevent the buildings from being protected to clear the path for the new bridge - a move that has been attacked as "inappropriate meddling" by environmentalists.

Historic Scotland, however, has resisted the pressure and listed the buildings on the old military base because of their "architectural and historic significance". This means they cannot be demolished unless consent has been granted under a special procedure, which can involve ministers.

The site at Port Edgar was bought by the Royal Navy in 1916 and used as a base for servicing torpedo boat destroyers during the first world war. During the 1930s, the barracks became holiday accommodation for poor families, and then reused by the Royal Navy during the second world war.

After it was closed in 1975, ownership passed to the Scottish Office and then the Scottish government, which held on to the base so it could be used to help build another bridge. The plan was to demolish the old naval buildings to make way for a new access road so construction traffic wouldn't have to go through South Queensferry.

But correspondence released to the Sunday Herald under Freedom of Information legislation shows this plan has been stymied by Historic Scotland. In April last year, the agency consulted on plans to list the buildings as "a rare and intact example of a naval barracks".

This drew a sharp response from Transport Scotland, which is behind plans for the new bridge. "We are concerned that any listing of these buildings at this time would be premature," wrote a senior transport official last May.

Despite this, Historic Scotland listed the buildings in September. They include five two-storey barracks, a guard room with cells, a boiler house, an S-shaped air-raid shelter and a latrine.

Malcolm Cooper, Historic Scotland's chief inspector, said the agency could not take account of possible future developments when assessing whether to list buildings. "This is an important part of military history,"he said.

He hoped detailed planning for the new bridge, now under way, would try to find ways of preserving the site. "Listing buildings flags up that they are of merit," he added.

Developers wanting to demolish listed buildings have to apply to the local authority for special consent. If consent is given, the matter then has to be referred to Historic Scotland, which can ask ministers to call for an inquiry.

Stuart Hay, head of campaigns at Friends of the Earth Scotland, said: "We are alarmed and disturbed Transport Scotland feels it is acceptable to attempt to coerce another public body to disregard its statutory conservation duties."

Bruce Whitehead, chair of the local action group, Queensferry Against Another Bridge, urged authorities to save the naval yard. "Ministers should not be allowed to by-pass proper democratic process by including the bridge under national planning framework legislation," he warned.