Will Norman Foster and Anthony Caro cross the Thames in a blaze of glory?

The Millennium Bridge, from St Paul's to the Tate at Bankside, would be a wondrous sight and a wondrous walk. It would also be the first Norman Foster work to have a dramatic presence in London.

It would be like a blade of light skimming the Thames, a footbridge 300 metres long and four metres wide from the riverfront steps of St Paul's to the new Tate at Bankside.

Slender as it is, this tightrope of a suspension bridge will have no Indiana Jones tilt and sway - the vibration, one quiver in three seconds, will be unnoticeable. Though anchored on two slender concrete piers, the Thames's two seven-metre tides a day will leave it unmoved. It will cost pounds 10m, a little more than building a four- storey office block; pounds 5m of it National Lottery money, matched by another pounds 3m from sponsors (Southwark Council has already put up pounds 1m). Subject to planning permission from Southwark and City Corporation of London it will open as a footbridge in the summer of 2000.

Designed by Norman Foster with the sculptor Anthony Caro and the engineer Chris Wise of Ove Arup, it emerged as winner of the competition run last year by The Financial Times in the face of 220 entries from all over the world, including the great Frank Gehry. You can see why they all entered - the chance to span the Thames at such a spot is a fabulous opportunity for an architect. Whether other people - local residents and councils and City types - are similarly enthused is another matter.

Bridges make connections. They bridge gaps, but some gaps are more difficult than others. Take, for example, the difficulty of ever getting things built when two planning committees from two highly contrasting boroughs have to reach the same decision: the richest borough in London, City Corporation, and the poorest, Southwark. People on either side of the river have been invited to public exhibitions of the winning scheme. They have studied the bridge proposals at RIBA and at St Paul's - at present the Millennium Bridge proposal is on view at the Design Museum in London - and many have willingly commended it.

Detractors are mainly in the City and mostly fretting about the view of St Paul's from the river. Actually, it opens up the view of St Paul's from the river but those passing under the bridge in a boat will lose sight of St Paul's for a nano-second when the concrete pier blocks it. An architect who has built a lot on the waterfront, Piers Gough, expressed his reservations about the need for a new bridge over the Thames at the English Heritage brainstorm session this summer. "Is there any doubt that it's a one-stop bridge bringing tourists to the new Tate?" he asked. As it will be the world's largest gallery and MORI predict over six million people a year between St Paul's and Bankside, that would be some "one stop."

In any case, Gough's interpretation is highly questionable. As David Bell, chairman of Pearsons, and chief executive of The Financial Times which sponsored the competition, is anxious to point out, the Millennium Mile on the riverfront to which the bridge would lead has plenty of "stops", from the County Hall aquarium at one end and the Globe and the Rose theatres at the other, as well as the Lambeth Palace gardens, the proposed wheel and the Lido for which planning application has been made, Waterloo terminus and a new Imax cinema. Besides, people from both sides of the river will walk to work across it. True, there will be an enclosed pedestrian crossing on the Blackfriars Bridge road and rail link nearby, but crossing that will hardly be an experience of the same order.

"The bridge will link two sides of the Thames at an historic point and give free access to the largest gallery in Europe. It will be the most permanent symbol of the millennium in the city. The Millennium Bridge is very beautiful and of our age. It will last," says Bell.

It will also be an opportunity, the first, for Norman Foster, the greatest British architect this century, to build something really eye-catching in the capital. The buildings for which Foster is famous are all abroad, buildings such as the waterfront Hong Kong and Shanghai Bank, and the Olympic telecommunications tower piercing the blue skies of Barcelona like a javelin. Admittedly London does sport his Sackler Gallery extension at the Royal Academy, and a gem of a building it is, but too small and concealed to be a landmark.

But these days it is not enough for architects to design the world's most beautiful buildings or bridges. They have to sell them as well. To Fine Arts, to English Heritage, the lottery funding, sponsors to match any lottery money, and patrons if they don't get either. All summer Daniel Libeskind has been on his feet at the Victoria & Albert Museum, waving an origami-like paper model of his Spiral to win over critics and talk through their fears on his proposed extension there. Stamina is what the V&A looked for when they told him his competition entry had won. The bigger the name, the bigger the hassle. So Norman Foster, who put together a slide show on the Millennium Bridge to present designs, has put in more performances than a star actor playing in repertoire at the National Theatre. Now his stand-in, Andy Bow from Foster Associates, delivers the lecture complete with laser pen pinpointing the slides. It starts off unthreateningly with brown sugar cubes balanced on pieces of paper and teaspoons across the void, which explains the relaxed way in which all three work together. Norman Foster recalls the thrill of "walking on water in a blade of light," which is the way that Anthony Caro infused them with the magic of the project in the first place.

As the project has evolved, and changes have been made to accommodate the judges and respond to public concerns, there have been many changes. First, the bridge was spanned in glass but people feared they'd slip, so now it is decked in teak. A backlit glass panel runs the length of the 300 metres, bank to bank, because that strip of light makes people feel more secure. Chris Wise had to address fears about the wind in mid- crossing on a bad day, so the bridge was tested twice in Canadian wind tunnels at fierce velocities and as a consequence the balustrading thickened and curved to deflect currents. Anthony Caro's sculptural entrances to the bridge came under review for wheelchair access - no changes in level or stairs - and unexpectedly, for their art. Nicholas Serota of the Tate and de Meuron, his architect for Bankside, didn't want the curvaceous cog-like Caro sculpture that was planned to stand on the sculpture forecourt of the new Bankside and anchor the suspension cables. So Caro wistfully replaced it with a boulder on that side and has set to work to design two gateway pavilions to mark the St Paul's side. With good humour, the Gladiatorial team have shown that they can listen and adapt the Millennium Bridge design. The only criticism that they have been unable to answer is that people may want to jump off it.

MILLENnIUM MILE

Those who cross this bridge from St Paul's (if bridge there be) will walk on to a South Bank packed with pleasures. In the Eighteenth century Londoners went south of the river for their amusements; momentarily they did so again in 1951 for the Festival of Britain but, since then, pace Ken Livingstone and Tony Banks's efforts, people have gone there exclusively for high culture. Come the millennium, however, it will, as this map shows, be irresistible in the variety of its pleasures