George Orwell, in Homage to Catalonia, described it as one of the most hideous buildings in the world. He thought the anarchists in the Spanish Civil War showed "bad taste in not blowing it up when they had the chance". They burnt Gaud's plans, however, causing headaches for his successors. Salvador Dali called the work "a tactile erogenous zone" and praised his compatriot "for his superbly creative bad taste".
Fierce passions are aroused by this astonishing building, with its towering barley-sugar spirals, each topped by a gorgeous, bulbous, fungoid flourish. These passions have dictated the church's zigzag fortunes down the years. In Spain's most "can-do" city, it is not laziness or indifference that has left unfinished a monument locals consider a symbol to match the Eiffel Tower. Even "Cobi", Barcelona's mascot for the 1992 Olympics, toted, Sagrada- like, four long loaves under his arm.
To host that event, symbol of its renewal, Barcelona rotated upon its axis; once "fleeing the sea", as the Catalan poet Joan Maragall lamented at the turn of the century, it turned to face and embrace it. A city capable of such revolution could surely complete even Gaud's wildly idiosyncratic vision. Or leave it, as Dali suggested, "like a gigantic decayed tooth, full of possibilities".
No, this vast and crazy enterprise has been riven by conflict from the start. Unlike the modern triumphs celebrated by the Riba prize, and the splendidly restored Gaud works planted about the city, the Sagrada Familia receives no municipal financial support. It is funded by private donations from Japanese enthusiasts and Catholic faithful who, to judge from the gathering speed of work in recent years, are getting richer or leaving fatter legacies.
The Catalan sculptor Josep Subirachs is in charge of finishing the work, commissioned by the architect Jordi Bonet, whose father worked with Gaud. "Gaud himself wanted his work to continue," says Bonet. "I know this from my father." But Subirachs' additions have provoked outrage, especially from Oriol Bohigas, the municipal architect hailed this week by Riba as one of the inspirers of Barcelona's rebirth.
"Architectural assassination, a pernicious joke," Bohigas thundered years ago, over efforts to continue Gaud's project. Subirachs' extravagant sculptures were "by far the worst he has ever produced", Bohigas reckoned.
Subirachs dismisses his critics as "barbarians" and says protests just "give me new impetus to carry on my work". After all, Gaud himself had few supporters in his lifetime. An austere, hermit-like man of fanatical religiosity, he began work on the Sagrada Familia in 1883 and devoted the rest of his life to it, living on the site, until the day in June 1926 when he was killed by a tram.
Hippies and free-thinkers who ventured upon Barcelona's melancholy gloom in the Sixties adored Gaud for his voluptuous, hallucinogenic excess. It is curious to think that the creator of this subversive ecstasy, who made stone appear not just soft but edible, was an inhibited and deeply conservative man. We crept up the spiral steps of the tower, peeped from ever smaller lookout holes and marvelled at words of faith carved upon what seemed a true stairway to heaven.
Mounted upon a structure of dizzying swoops and spirals, stone-quarry surfaces and molten-wax effects, Gaud's figures of the Nativity are utterly naturalistic. He took plaster casts of his craftsmen to make them into saints; photographed women playing harps alongside mirrors to ensure that his angels were in perspective; took casts of a donkey, geese, snails (which, 10ft wide, creep round an arch 50ft from the ground) and even stillborn babies, to depict the Slaughter of the Innocents.
By contrast, Subirachs' figure of Christ suspended from a steel girder, and those of Roman guards in Darth Vader masks, have been condemned as "early-Sixties kitsch", his additions dismissed as "Disneyland" and "a birthday cake". A protest was organised in front of the church some years back, at which hundreds of artists and architects hurled insults such as: "Subirachs, you are doing the work of the pigeons" and, cruelly, "It's a pity there are no more trams".
"I would be very happy if the church were never finished," said Bohigas this week, after the Riba prize was announced. "I'm a great admirer of Gaud, but to think you can imitate him is a terrible error, a grotesque exercise in bad taste."
In the Fifties, Bohigas organised an international campaign, backed by 1,000 architects including Le Corbusier and Gropius, that appealed for the work to stop. "It is impossible to reproduce architecture according to an aesthetic and a technology that have been totally superseded," he says today.
But the church's devotees are undeterred. They even want to make its creator a saint. Last August, the Cardinal of Barcelona, Ricard Carles, set in train the long process of beatification, saying that Gaud should be considered for sainthood because of his "profound and constant contemplation of the mysteries of the faith". Gaud's champions are aiding this with claims of miracles: one devotee said that she painlessly passed a kidney stone thanks to the great man's intercession. An architect attributed to Gaud his winning of an important prize.
Josep Maria Gordi, a young architect who walked to school in the shadow of the Sagrada Familia and whose studio overlooks it today, says: "The best solution, in my opinion, would be to destroy everything built since Gaud's death and leave his achievement in the midst of a green space. This is not a collective effort, like a medieval cathedral. You can't just add bits on. It's as ridiculous as trying to complete a painting by Picasso."
But those determined to finish the church are an important conservative force whom any local politician would hesitate to confront, Gordi says. "I think it's better to let things lie for the moment, because before long the project's internal contradictions will reach a crisis."
Bohigas is keen to prevent work going any further. "We have to take a definitive decision to stop. They can't finish it. The space into which Gaud planned to expand has already been built on. You'd have to knock down houses to complete it."
With the city again in the public eye, wouldn't this be the moment to revive his campaign? A pause. A chuckle. "You've just given me a good idea. Maybe we can end this horrible cock-up once and for all." Stand by for renewed polemic over Gaud's best-known work.