The £23m scheme will see the Abbey's small plain pyramid roof being replaced by a more decorative corona.

A new museum and exhibition space showcasing historic treasures and artefacts previously unseen by the public will be housed in the upper gallery, known as the Triforium, while an education centre and a café will provide improved visitor facilities.

The Dean said: "Westminster Abbey is recognised by people in every part of the world. The rich and fascinating story the Abbey tells, of Church, Crown and State, goes back well over a thousand years. So many occasions of national and international significance throughout that time have left an indelible mark.

"We want all the people who visit the Abbey as worshippers or tourists to be able to experience that story.

"Opening up the Triforium to display many of the Abbey's treasures, developing the education centre for school groups and providing the Cellarium café for our visitors' refreshment will all contribute to opening access to the welcome, wonder and worship which are the marks of the Abbey."

The 2020 Vision exhibition at the Abbey's Chapter House is open for public viewing and comment until the end of September.

It is hoped the project will be completed by 2013 to mark the 60th anniversary of the Queen's Coronation at the Abbey.