A sneak preview

The power and the glory: 'It doesn't matter whether Bankside is a museum, a gallery, an ace caff, a cathedral or a bookstore. At one hundred and thirty-odd million, it's a snip. A bargain. Those fools who sanctioned the Dome must look at Tate Modern and be utterly ashamed of themselves'

I sit in my car and look at the solemn bulk of Bankside on a drizzly morning. Picking my way between JCBs, I make my way into the building that will soon be Britain's most astonishing art gallery. Tate Britain opened last week on Millbank; Tate Modern has five weeks to go, but the excitement is tangible. Museums are thrilling while still being built, the motivating idea intact, not yet contaminated by compromise. The loading bays are full of fine-art packaging and building materials suspended in a curious material democracy. The interiors of the lift cars still have their protective coating. The optimistic sheen of stainless steel remains immaculate. A Mondrian is visible 100 metres away.

It is clear already that the energy and scale of Tate Modern will make Tate Britain look quaint. This is the future. But first, some history. The supply of electricity began in London in 1878, when Whistler was painting his moody views of the Thames beaches (just along from the spot on which the wreck of Battersea Power Station now slouches). By about 1890, electricity was widespread, and soon power stations would become noteworthy urban landmarks, with a distinctive style of their own: brick, massive, faintly Amsterdam school with their severe details, but elegantly impressive none the less.

The most complete expression of that vernacular style was Sir Giles Gilbert Scott's Bankside, completed in 1950. It was an extraordinary building, a familiar sight across the Thames from St Paul's. But its working life was short, and by the Nineties it was sitting derelict, smack in the middle of a South Bank that was promising to develop into a continuous cultural strip, from the Festival Hall to the Design Museum.

Now, in an astonishing conversion, the old Bankside has been reborn. And next month it re-opens to the public as the rather too cutely named Tate Modern. The conversion is the work of the Swiss architectural firm of Jacques Herzog and Pierre de Meuron - born the very year that Bankside went on to the National Grid, and hitherto best known, if known at all, for a wonderfully dry engine-shed in Basle. Local contenders in the competition - David Chipperfield, for instance - were justifiably disappointed to have been refused the opportunity to create an international landmark in their home town, but only a curmudgeon would deny that Herzog and de Meuron have done a superb job.

Visitors now approach from the west, down a heroic, angled ramp, which swoops underground into an excavated area that extends what was once the cavernous turbine hall into Hades. Bravely, this space is being left almost empty. Depending on your point of view, the single work of art on display in this Grand Canyon of spatial bravura, a looming Louise Bourgeois tower sponsored by Unilever, will be a culminating feature or a trivialising distraction. The space itself says quite enough about the modern experience without the intrusion of a work by an artist described by Brian Sewell as a sad old bat. In many art museums - the east end of Washington's National Gallery or the Bilbao Guggenheim, for instance - the building is much more impressive than the paintings or sculpture. That threatens to happen here. In one sense, Bankside is the opposite of Paris's Pompidou Centre, where everything is exposed on the outside. At the Tate Modern, everything is revealed on the inside. It is awesome. On top, Herzog and de Meuron have added a two-storey glass pavilion, which houses a restaurant and dining-rooms. Below are three levels of galleries. Eventually one will be used for temporary exhibitions, but right now all 80 galleries are devoted to the permanent collection. The old Tate at Millbank could show only 15 per cent of its stock; Bankside can put half its collection on display.

As I walked around, I was reminded time and again of Emile Zola's description in Au Bonheur des Dames (1883) of Paris's new department stores: "Seen in perspective, with the show windows on the ground floor and the plate-glass mezzanine floor windows, behind which all the internal life of the departments was visible, it seemed... to be an endless vista... a cathedral of modern business."

That was then. Now the museum is the distinctive building of our age, the one in which all the symbolic values of our civilisation are invested. Like Paris's shops, it is a cathedral of modern business, a symbol of all our values. If that means that the sanctity of the cathedral is sometimes compromised by commerce, then that is an authentic modern experience. Kurt Vonnegut was going too far when he said, "Art is a conspiracy between clever parasites and millionaires to make poor people look stupid," but it is a truth universally acknowledged, that a man in possession of a good fortune must be in want of a museum. Sir Henry Tate was a sugar refiner. Solomon R Guggenheim was in copper.

The commercial context can be extended: Andy Warhol said that stores would become museums and museums would become stores. Quite. As Picasso's biographer, John Richardson, observed, the first thing you notice in New York's Metropolitan Museum is the ping of cash registers. The architect Jacques Herzog is entirely comfortable with the idea that architecture itself is transcending the strictures of philosophy and engaging in the more volatile and immediate world of fashion and design. But how are the Tate's curatorial obligations accommodated here?

Bankside liberates the Tate from some of the absurdities forced on it by the dual roles at Millbank of being a sort of antiquarian British Valhalla while also having to represent the international phenomenon of modern art. But Tate Modern is not without its own intellectual conflicts. Its galleries are predominantly devoted to the display of the permanent collection, a representative survey of 20th-century art according to the scriptures of modernism. Instead of being arranged chronologically or nationally, the works are displayed thematically, so there are astonishing juxtapositions: a Richard Long against a Monet is just one example. Such an arrangement maintains a freshness, but the suspicion - gaining force ever since New York's Museum of Modern Art hijacked modernism in the Thirties - is that fine art is, if not yet a closed book, one that is coming to the end of its storyline. The hanging in Tate Modern may be adventurous, but the curatorial assumptions about reverence and respect are in evidence here as surely as if it were works by Chardin or Velázquez on display.

At the same time, Tate Modern has to cope with the fragmented and anarchic condition of contemporary art, the stuff that people who categorise themselves as artists produce today. His detractors have identified at the Tate what they call the Serota tendency, an inclination to incubate pranksters and mountebanks masquerading as artists. Certainly, the Tate has its own dodgy salon des acceptés. Tracey Emin is typical: she has a genius for outrage, but no taste for aesthetic value, at least not of the conventional sort. You do not need to be a conservative to wonder what purpose, other than the pursuit of notoriety or celebrity, is served by a sleazy old divan and dirty knickers.

Still, it was a delicious experience to walk around Tate Modern a month or so before it opens. There were marvellously surreal encounters with crates marked "Picasso" rubbing-up against electric dollies, rolls of gaffer tape and cursing electricians. In such a poetic environment, everything - a discarded styrofoam cup, a strip of bubble wrap - becomes an objet trouvé. My guide carefully explained that one giant rusting steel joist was, in fact, part of a Joseph Beuys installation, not a remnant from the days of the Central Electricity Generating Board.

But that, of course, is the beauty of the modernist conception world. It's art because I say it is. And even more so when it is in a gallery, especially one that jostles with the Pompidou to be the biggest in the world. Tate Modern weirdly combines elements of high culture (a reverence for the creative process) with those of low (vicarious contact with stars). But this is good. Just one doubt lingers. Art, as Picasso knew, must be subversive. As he said: "Once art becomes official and open to everyone, then it becomes the new academicism." Actually, I wonder if that is the real Serota tendency: academicism. Here at Tate Modern, art is branded, institutionalised, commodified.

But that's a quibble. The building is magnificent. Tate Modern, judged on any basis, but especially on an artistic and intellectual basis, is immeasurably more worthwhile than the melancholy Dome. It doesn't matter whether it is a museum, a gallery, an ace caff, a cathedral or a shop (it does have the largest art bookshop in Europe). It's an investment whichever way you look at it. Symbolically, a new footbridge links it to the City. Never mind its cultural gravitas, its economic influence on the South Bank demonstrates the cash value of art just as much as $26m-worth of sunflowers do. At £130m-odd, it's a snip. A bargain. The fools who sanctioned the Dome must look at Tate Modern and be utterly ashamed of themselves. Electricity generation is going to start again. Southwark is now on the international grid.

Tate Modern opens on 12 May. For information, tel: 020-7887 8008