Who dares upstage a genius?

The National Theatre's original architect is furious at the way his vision has been updated. Yet the new design duo responsible claim to be two of his most loyal fans.

Act Two of the National Theatre opens to rave reviews this summer. The makeover of the public spaces within and without the original building by the architects, Alan Stanton and Paul Williams, is popular with everyone - except the man who produced the building in 1973, Sir Denys Lasdun. He doesn't like it. In fact, he's furious.

"Stanton Williams have badly damaged the architectural composition of the building with the new entrance," he said. "I'm not against changes, but those changes have got to be in sympathy with the spirit of the building. The new entrance isn't."

His objections centre around the bookshop, which has moved, along with the booking office, from the dimly lit recesses, right up front of house on either side of the entrance. In the days when the ticket office was on the far side right-hand corner, lit up at night like a beacon, the bookshop was no bigger than a couple of counters in the middle of the Lyttleton foyer. As the pressure on both spaces grew, they moved nomadically about the building. Now that they have taken up a permanent position at the entrance, Sir Denys believes the bookshop obstructs the flow of people. He doesn't object to the ticket office, which he concedes is an operational move. But he is convinced that the presence of the book shop at the entrance destroys any sense of coherence of the movement of the public.

"It's not malice that made them place the bookshop there. The decision was just driven by revenue. That's my conjecture - I'm not in communication with them. I only get cross and disappointed because, after all, it is a Grade II listed building and everything we said to them has been ignored, even though I have a letter from the Chairman promising me that the integrity of the building would be respected."

Sir Denys believes that the bookshop location ruins his architectural composition of the terraces descending rhythmically from the top of the building to the ground. The building is set back to let people on Waterloo bridge have a longer view of St Paul's from the National; now it is blocked by the bookshop but only from one angle. If anything, the building now faces up to its riverfront position by pedestrianising the river and building a terrace over the tarmac.

At the time Sir Denys designed the National, he said: "I call these terraces, which are very horizontal in emphasis, strata. It's a geological term which goes very well with concrete. These platforms and terraces are public spaces, domains, an extension of the city."

Though not always a popular one. Prince Charles memorably dismissed it as a concrete bunker. But now English Heritage grade-lists Sir Denys's buildings and he is moving into blue plaque land, there is an acknowledgment that he is the major British architect of the 20th century. A book, Denys Lasdun: Architecture, City, Landscape, by William Curtis, (Phaidon, pounds 45) reveals the patient sleuthwork Sir Denys did in the Sixties to reconcile the needs of the directors and actors with his architectural admiration of classical theatres, particularly the oldest surviving and best preserved example, at Epidaurus.

He took patient notes of every ego-charged conversation: Peter Brook told him he found it exciting to contemplate theatre on a bombsite in Bethnal Green, long before he staged The Mahabarata in an old tram-shed in Glasgow. And Benjamin Britten told him that people need a fairytale setting in order to be able to endure the truth. Britten favoured churches as settings because "the building took people out of themselves. No talking, no applause." Not even his detractors would deny that at the National, Sir Denys managed to take people out of themselves and give them a transcendental experience. You simply cannot divorce the form from the contents. But 35 years after it was conceived, the National Theatre has reached a mid- life crisis.

Black-and-white photographs taken at the opening of the National Theatre reveal a very late-Sixties story. Beehive hair-dos, long dresses with stoles and fur trims. Men perhaps not in black tie, but still all wearing ties none the less. If the photographs were in colour you would see the mustard-coloured Formica at the bar, the hessian-like sackcloth on the walls, the purple carpet wall-to-wall. And the shaft of orange light beamed down into the Lyttleton foyer along prismatic columns of ash-coloured cement.

Now only the purple carpet remains, a replacement following Mary Lasdun's original design. Lighting designed by Maurice Brill, who set up the original system, brings out the "warm, furry feeling" of the concrete interior. Bars have been soundproofed from the auditoria by Ove Arup.

Stanton Williams are adamant that they recognise Lasdun's genius - "Flannel," says Lasdun dismissively. "If you admire something, you don't damage it." Like all good plastic surgeons, Stanton WIlliams have used nip and tuck to subtle effect while recognising the basic bone structure. Nothing too startling, just bringing back the cement to a silvery burnish by giving it a facial scrub and replacing tired, yellow lights with flattering theatrical lighting.

Its craggy, concrete face has not aged gracefully. Visiting Switzerland and France, where concrete cleaning techniques are up to scratch - "we're good on stonework in Britain for historic monuments but not on cleaning cement" - Stanton Williams pioneered a cleaning system which will be invaluable for all modern buildings in Britain. All the cabling in the conduits was replaced and the stains washed off with an eco-friendly solution. At night the building lights up to a soft glow.

More challengingly for Sir Denys, they have brought the building back on to the riverfront by stopping the road at both ends of the National's facade. Where there was a busy road and a taxi drop there is now an elegant square, or piazza, as they call it.

"The road running around the National like a moat cut it off from the river," Alan Williams says. So they have restricted access through it to the disabled. Car park entrances are confined to the back and truck deliveries to the side, along with taxi drops.

Modernising London's most challenging modern landmark isn't a job any architect would relish. But Stanton Williams have taken into account changing audiences and their needs: disabled theatre goers, people who like to buy books along with their tickets, and who want to eat in a well-run restaurant.

Not one performance was lost in the six years of work. Shows went on around the building works, a lot of which were conducted at night. Stanton Williams are pleased that much of what they have done for the pounds 10m is not immediately obvious. This isn't as self-effacing as it sounds: they see themselves as sculptors working with space and light to interpret the needs of the users.

Every building has at its heart a penetrating idea that expresses itself through every part and every detail. That notion is pure Lasdun but Stanton Williams believe it to be true. Maybe one day they will get together for a meaningful dialogue. If only Sir Denys can stomach it.