You have to remember the complete shock of walking in. There wasn’t anywhere like this in London. In Britain! The space!” Robin Day – at 92 still Britain’s most esteemed furniture designer – is momentarily lost for words. The Royal Festival Hall still affects him, 56 years after it opened in 1951. Then, he and his wife, textile designer Lucienne, were the Festival of Britain’s golden couple. Robin, design’s young Turk, had designed the Hall’s furniture and auditorium seating; Lucienne was to launch her best-selling Calyx fabric at the Festival.

Sir Terence Conran – then a green, 19-year-old sprite, working on the Festival’s pavilions for his first job after design school – remembers the gala opening, his nose pressed up against the Hall’s glass walls. “It had been a long time since London had been so glamorous. Everything put up then was so shabby. Yet here was this gleaming, immaculate monument – for us!”

There wasn’t a public building like the Festival Hall back then. “This was Fabianism triumphant,” says Conran – the symbolic heart of Attlee’s postwar welfare state consensus, with its central philosophy of “fair shares” for all. The site chosen in 1948 for the Festival by cabinet minister Herbert Morrison may have seemed, to him, a dispensable, grimy neighbourhood of wharves, warehouses, lockups and terraced houses. But, though overlooked by most Londoners, it also happened to be the geographic centre of the capital, the perfect riverside stage for this symbolic act of transforming “old London, old Britain” into new.

“You can’t imagine how drab London was then,” says Conran. “We all wanted to go to New York where they listened to jazz. The modern just didn’t equate with London.” The Hall brought modernism belatedly centre stage in Britain, several decades after it had been born in continental Europe, thanks to its group of young architects – few over 40 – flushed with architectural and social idealism, and, says Conran, “eager to prove to the world they were more than just demobbed soldiers”. Robert Matthew, fresh new chief at the London County Council Architects Department, proved a sharp leader, ably abetted by Leslie Martin, the LCC’s deputy chief architect in charge of the Hall’s overall design, Peter Moro on interiors, both bred on Le Corbusier and Scandinavian social democratic “humane modernism” – with LCC stalwart Edwin Williams for the business end. Herbert Morrison created a special committee to fast-track decisions, while Martin and Moro got together a team of students and friends such as Day and the young architect Trevor Dannatt.

“We were idealists,” says Dannatt. Day recalls “a constant feeling of continual, organised panic. It was so exciting, though, working together to this common end, inspirational even.”

The challenges were fourfold. First, building quality architecture at a time not for nothing dubbed the age of austerity, with a total ban on construction save for homes, schools and factories and severe materials shortages. Every detail down to the doorknobs had to be designed from scratch, not just to create the modernists’ ideal of the “Gesamtkunstwerk” – or complete work of art – but because Britain’s factories simply weren’t yet making building components on any scale. The bigger challenge, though, says Day, was that “it had to be done in a tremendous hurry. There was never time to sit back and consider”. And it was done during the wettest winter and spring since 1815, the building site a mudbath.

The two design challenges, though, led to the building’s stroke of genius. In answer to how to build a 3,000-seat concert hall on a tiny site next to a mainline railway, Leslie Martin came up with his famous “egg in a box” design, the auditorium “egg” sitting on delicate columns wrapped in a cube. The space surrounding the auditorium protected the audience from the unwelcome accompaniment of the 7.47pm from Charing Cross.

It also gave London one of the greatest public spaces in the world: the Hall’s foyer. There were few spaces in postwar London where social classes could mix; theatres, indeed, often had separate entrances, stairs and bars for those with differing tickets. The Hall’s foyer, though, was free for all, expressed in a landscape of terraces, balconies, stairs and landings. But almost as soon as the Hall opened – after the initial buzz of the Festival of Britain – the mood changed. The “humane modernist” architecture that the Festival introduced to Britain was denounced by a new generation – later dubbed the new Brutalists – as twee.

Almost immediately, the building seemed dated. The site atrophied. When the LCC finally did come up with a plan, in the early 1960s, to create a cultural quarter on the Festival site, it was realised in the newly triumphant architectural clothes – Brutalism. The Hall was first wrapped in the completely new façade we see today, replacing its “twee” 1950s decoration with something starker, more abstract. A second and third auditorium – the Purcell Room and Queen Elizabeth Hall – were built next door and the Festival Hall wreathed in pedestrian walkways so that Martin’s original street entrance to the southeast – towards Waterloo – became obsolete, along with the rhythm of the “architectural promenade” he’d so carefully constructed. It was, says Dannatt, “an absolute tragedy. They completely misunderstood the building.” “I couldn’t bear to visit it,” agrees Robin Day.

Left a decrepit ghost of a long-dead age, its magnificent flowing foyer spaces stuffed with a shanty town of shops, bars and restaurants, the RFH’s original power leaked away.

Its Lazarus-like rebirth this weekend was conceived at its lowest ebb. In 1992, the South Bank’s then chief executive, Nicholas Snowman, invited Martin and Moro for lunch and soon after appointed Bob Allies and Graham Morrison, students of Martin at Cambridge University, as house architects.

With tears in his eyes, Peter Moro said to Morrison: “If you could at least recover the foyer spaces before I die, I will die a happy man.” Sadly he died in 1998, Martin in 2000. “But,” says Dannatt, “at least now they can rest in peace.” The new Hall has cost nearly £115 million but looks fabulous: the central auditorium has been recast, and the public areas are as open and uncluttered as they were when the Hall first opened. Perhaps we can now treat the greatest gift this architecture bestowed on us with the respect it has for so long sorely deserved, but, until now, so rarely received.

Royal Festival Hall Revival, an exhibition revealing how the landmark building has been restored, is at RIBA Architecture Gallery, Victoria & Albert Museum, London SW7 (020-7942 2000, www.vam.ac.uk), June 21-October 14

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