Why my National Theatre should remain untouched

There are plans afoot to prettify the National Theatre, Sir Denys Lasdu n's controversial masterpiece. He is far from pleased

Public buildings must accept some degree of change and adapt to new circumstances but, especially in a Grade II* listed building like the National Theatre, such changes must avoid damaging the coherence and integrity of the architecture.

Plans to demolish the lower terrace and replan the main entrance are little short of acts of vandalism. The essence of the original design is what I call its stratification, by which I mean that the overhanging terraces of the building descend in a series of rhythmic layers down to the river. The terrace and the colonnade at ground level establish a human scale and avoid a cliff-face monumentality. The proposed removal of the lower terrace would effectively destroy the whole architectural composition ofthe building and its relationship with Waterloo Bridge, as well as interrupting the pedestrian flow enjoyed by the public from the bridge to the theatre's riverside promenade.

These are very damaging changes. They are crude, diminishing both the integrity of the building and the experience theatre-goers and passers-by have of it. The blocking up of the main entrance porte cochere by a new box-office and bookshop all contributeto the destruction of the axial approach into the theatre. The 40ft high "cathedral" window inside the Lyttelton foyer, at which so many people like to sit before performances and between acts, is to be cut in half and the view of Somerset House opposite obliterated.

The National Theatre is still a controversial building, though I am told people are beginning to acquire affection for it. I am aware that many find the structure tough and uncompromising in its aim to produce a building that is of intrinsic architectural quality without prettification. It would help enormously if it were cleaned - which is not much to ask after 25 years.

The building overlooks King's Reach on the river Thames - the most magical bend of the river, offering panoramic views of the city. Spend a bit of time looking at it and using the building and you will find that I have tried to make it connect with London on many levels. It is a very respectful building; it's also one of those buildings, I suspect, that wins people over in the long term.

Five years ago the theatre asked me to write a report, "A Strategy for the Future", which demonstrated that changes could be made to accommodate most of the theatre's wishes for the future without damaging its architecture. The last thing I expected fromthis report was to be thanked for my advice and have it ignored and to see the theatre's new architects taking an axe to the building, hacking bits away in reply to short-term decision-making by the theatre.

The Royal Fine Art Commission shares my objection. People from all walks of life have been encouraging and have written to Lambeth council, the planning authority, to express their outrage.

There are now plans to restore the Royal Festival Hall back to its original 1951 state, at considerable cost to the public. The National Theatre is also a public building of national importance and with an international reputation. The present proposals deserve to be debated in a public inquiry before it is too late.