The plan, which involves radical changes to one of Britain's top tourist attractions under consultant architect Sir Norman Foster, also means that campaigners to keep the Round Reading Room as a centre of study for British Library cardholders, its original purpose, have lost their battle.
It also flies in the face of the recommendation last week by the House of Commons heritage select committee that the reading room should maintain its present function for British Library readers on its present site.
Instead, when the library moves out of the museum to St Pancras, the reading room, in the words of British Museum director Dr Robert Anderson, will be 'divided in two', with one part an information centre about galleries and exhibits elsewhere in the museum, and the other part a reference library, which would be modelled on a normal public library reference area, with books on shelves on the wall rather than being specially delivered from stores.
In its new guise, the historic Round Reading Room will lose some of its reverential atmosphere. Dr Anderson and the museum's trustees emphasised that the new plan for the reading room would keep its famous design and character, but would give unlimited access, as opposed to being 'just for the privileged few' who hold British Library reader cards.
Last night, Brian Lake, secretary of the Round Reading Room Regular Users Group, said: 'This proposal makes no sense. It is a reading room and we now have the heritage select committee saying it should stay the British Library reading room for perpetuity.'
He added that campaigners, who include Michael Foot, Lord Carrington, Lord Jenkins, Lord Thomas of Swynnerton and the philosopher, Sir Isaiah Berlin, would continue to press for it to retain its function.
At the centre of the 10-year plan - the most ambitious development undertaken by any museum in Britain - is the Great Court, the grand inner quadrangle conceived in the 1820s by Sir Robert Smirke, the museum's architect, and the size of one and a half football pitches. This inner courtyard was later obscured by the construction of the Round Reading Room and its associated book stacks.
The departure of the British Library, which will release 40 per cent of the museum's building in Bloomsbury, will give the opportunity to restore something of the architect's original concept, and, for the first time, will allow the general public easy movement at ground level throughout the museum.
Key features of the development plan are: restoration of the Great Court by demolishing the redundant book stacks outside the Round Reading Room; and refurbishment and redecoration of all the great public rooms on the ground floor, including the 100-yard-long King's Library.
The plan also provides for the most technologically advanced education complex in Britain, including new lecture theatres, seminar rooms and other facilities, and the return to Bloomsbury of the ethnographic collections moved to the Museum of Mankind in 1970.
Sir Norman's winning design beat a shortlist that included Arup Associates and Rick Mather Architects.
Sir Norman said: 'Enclosing the inner courtyard with an elegant roof responds to the Smirke facades and reading room. This will provide protection from the elements and transform the courtyard into a central focus for the museum. By constructing ramps around the reading room, a pleasurable dialogue between old and new can be established, leading the visitor to the upper-level galleries. Vital new accommodation within the courtyard is provided in elliptical form, a considered sculptural foil to both the rectangular courtyard and circular reading room.'
The plan faces two hurdles. First, there is still no firm date for the British Library to leave the museum and move into its much-delayed new building. Second, much of the money will have to come from the Millennium Commission, funded by the National Lottery, which will face many other demands. But Lord Windlesham, chairman of the trustees, and Sir Claus Moser, chairman of the BM's Development Trust, were confident the money would be raised.
The architect Michael Hopkins, a trustee who was on the selection panel, said: 'At the moment the first thing you get is the restricted access of the reading room. We need somewhere for the children to be able to write up their notes and eat their lunch. And we need to clarify the building and make it comprehensible. Visitors need to know where they are and where they want to go]