St Paul's has hosted many state occasions

The internal cleaning and relighting of the cathedral will take four years and will cost �10.8m.

It has been funded by an "outstandingly generous" donation from a single family, cathedral authorities said.

The internal cleaning aims to tackle 300 years of heavy use and air pollution.

In 1709, as the cathedral approached completion, the architect St Christopher Wren arranged for the entire interior to be painted "three times in oyle".

By the mid-19th Century, the painted surface would have been flaky and darkened with age.

Between 1873 and 1876 it was removed but an attempt to clean the exposed stonework in 1935 ended in failure because no suitable alternative was available.

Wren's vision

The current work began in May 2001 following extensive research.

The Dean of St Paul's, the Very Reverend Dr John Moses, said: "We will be able to show, for the first time in nearly 300 years, Wren's vision for the interior of the south transept revealed in the light and shade of the cleaned stonework."

Sir Christopher Wren was commissioned to design the cathedral in 1668 - two years after the Great Fire of London destroyed the old St Paul's.

Problems with the great architect's plans delayed the start of work for more than 20 years but the building was finally completed in 1710.