The site contains three properties, two of which - Corfe Castle, and the chalk pillars of Old Harry Rocks, at Handfast Point, - are in Dorset. The third, Petworth House, in West Sussex, is a magnificent late 17th- century mansion and beautiful park, landscaped by Capability Brown and immortalised in Turner's paintings. The house contains the Trust's finest collection of pictures, with works by Turner, Van Dyck, Reynolds and Blake.
Using a mouse, virtual viewers can explore panoramic scenes and see the landscape from a 360 degree angle.
The majestic ruins of Corfe Castle, which controlled the gateway to the Isle of Purbeck and was an important stronghold from the time of William the Conqueror, can be viewed from a hilltop looking down on the site or from a vantage point nearby. It is also possible to "zoom" in closely and inspect the brickwork of the ruins.
The online move is a gamble for the Trust. Although it admits that it is possible the website may be a substitute for visiting the sites in reality, it hopes the internet presence will have the opposite effect, and encourage people actually to go and see the buildings. "You can never get the full experience of a property from seeing it on screen, but it is a concern," a National Trust spokesman said.
"We have created the site as an educational tool for children who may need to study certain buildings for history but are not able to go and visit them.
"I hope that we are not a nation of such complete couch potatoes that people will not be bothered to go and see the real thing."
Each property costs around pounds 1,000 to put on the Internet and there are plans to add more with ambient sound.
"We won't be putting in birds singing or aeroplanes rushing past overhead but perhaps a reading of a poem that was inspired by the site or a piece of music that was composed nearby," said the spokesman.
Richard Pollard, secretary of the campaign group Save Britain's Heritage, welcomed the National Trust's online initiative, saying he hoped virtual visitors would be inspired to visit in person. "Undoubtedly a number of these properties do suffer from intensive visiting but I think it is unlikely that the National Trust would try to reduce the numbers by diverting them to a website," he said.
"First, they are preserving these buildings so the public can see them - and see them in reality - and second, they would not want to lose the revenue which comes from the souvenir and tea shops."
The National Trust's virtual tours are at: www.nt-educa tion.org/virtualviews/indexReuse content