St Paul’s Cathedral is to give unprecedented access to rarely seen areas of the 300-year-old building, to lessen the serious damage being caused by millions of visitors.

To ease congestion in the nave, where memorial stones of Nelson, Turner and Reynolds have been worn away by people walking over them, visitors will, for example, be redirected to the trophy room, above the north aisle of the nave.

There they will find the 13ft (4m) great model of the 1670s, Sir Christopher Wren’s original vision — the cathedral he would have built if it had not been rejected, partly because of its resemblance to St Peter’s in Rome. Designed on a Greek cross plan, where all the arms were equal, and adorned with cherubs’ heads, flowers and festoons, it was dramatically different from what he eventually built. He was forced to abandon what most architectural historians believe would have been an even more impressive design.

St Paul’s, as one of London’s ten most-visited tourist attractions, attracts almost two million visitors a year. While stiletto heels are gouging out chunks of the marble and rucksacks have been known to knock out mosaics and bits of monuments, human breath is also changing humidity levels so acutely that they could affect the roof’s stability, it is feared.

An urgent need to redirect the flow of people is being addressed by a grant of £250,000 from American Express and the World Monuments Fund (WMF), announced yesterday.

The cash will also enable St Paul’s to open to the public the spectacular view of the nave from the upper level, above the western door — the only way of seeing the nave, culminating in the altar, as a single view.

Jane Fawcett, the architectural historian, said that opening extra areas would not make a huge amount of difference to the wear and tear.

She called instead for the cathedral to ban stilettos, shopping trolleys and buggies, which are all creating damage that is “very visible”.

She said: “A lot of the inscriptions that were legible ten years ago have become illegible.” Only from the cathedral’s own records do we now know who is buried where and what the inscription said, she said.

The big brasses in the crypt are also suffering from wear and tear, she said, adding that the cathedral’s decision to introduce entry charges has not deterred visitors.

She said: “People are determined to come. It is one of our greatest buildings. The only way to improve the situation is to control where people go, rather than increasing the availability of space. Those frightful, sharp stiletto heels are incredibly damaging. They really gouge out the stone. Those ought to be restricted.” Covering the marble with carpets and rugs does not help as grit gets underneath them. The humidity levels could “literally shake the roof”, Mrs Fawcett cautioned.

Jonathan Foyle, chief executive of the WMF, agreed with her concerns, but said that the cathedral could not afford to limit the numbers because it depended on ticket sales.

Founded in 1965 the WMF has conserved more than 450 projects in more than 90 countries. It works with local communities to identify and save important heritage. Every two years it issues its list of 100 most endangered sites, a global call to action on behalf of sites in need of intervention.

The grant to St Paul’s will also help to create educational programmes. A new exploration centre will show how architectural, historical, scientific, musical, and religious themes relate to the life of the cathedral.

Guide to good visiting

How visitors can help to protect important sites

— Visit popular attractions outside the usual times of day or year when they are less crowded

— Minimise the flash and lighting on photographic and video equipment

— Wear rubber-soled flat shoes to help to protect floors and stairs

— Stay on marked paths, do not touch walls and dispose of litter properly

Source: World Monuments Fund

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