Sweden and Denmark are frosty in more ways than one. But next month things will have to change when a new bridge re-establishes a link lost to history

Danish television is currently running a series of commercials whose message is not immediately apparent. For TV in Denmark, this is unusual. In one of them, a herd of elk stampedes through the streets of Copenhagen. The animals find their way to the gates of the Royal Palace itself.

Danish television is currently running a series of commercials whose message is not immediately apparent. For TV in Denmark, this is unusual. In one of them, a herd of elk stampedes through the streets of Copenhagen. The animals find their way to the gates of the Royal Palace itself.

Elk? In Copenhagen? Even the impassive Royal Guards on sentry duty are forced to bat an eyelid at that. Because as every Dane knows, they don't have elk in their country, let alone right outside their holy of holies. Elk are Swedish. "Prepare yourself for many new experiences," says the voice-over, "now that the bridge is coming."

For the millions of people within easy reach, the opening of the Oresund Bridge on 1 July will mean a radical change in their lives. And for countless others who have decided against exploring Scandinavia because of the cost or inconvenience, the bridge will make them think again. Or, at least, that's the plan. And it explains why Sweden and Denmark have staked so much money and expertise on less than 20 miles of new road and railway.

Half of that distance is taken up by the bridge, tunnel and artificial island that connect the two land masses. The dimensions, looks and engineering of the enterprise are stunning, but the implications of a permanent link might be greater still, altering the dynamics of northern Europe. Continental road maps will have to be redrawn; railway timetables rewritten. For the first time, it will be possible to drive uninterrupted (except to fill your tank and pay your tolls) from the arid shores of southern Portugal to the sub-Arctic crags of northern Norway. From a British perspective, the advent of the bridge will open a continuous road and rail link between Caithness and Helsinki.

And as if that weren't enough, Denmark and Sweden - whose relations for most of the last 480 years have ranged between polite frostiness and downright hostility - are joining forces to put a brand new place on the map (or at least one you've never heard of before) with a bridge at its heart. But this is no ordinary bridge.

Unlike a skyscraper, whose height is determined chiefly by the skill and imagination of the architect, and the wealth and ambition of the city that commissions it, a bridge is never any longer than it needs to be. The Forth Road Bridge is a few feet longer than the Severn Suspension Bridge. That doesn't make it a superior bridge: it merely tells us that the Forth Estuary at the crossing-point is slightly wider than the River Severn. Span alone is not a yardstick of greatness.

In the same way, the Oresund Fixed Link, which is about to join Sweden and Denmark for the first time since the Ice Age, cannot be measured by the wow factor of its dimensions, even though it's the largest construction of its kind in the world, and the bridge alone is five and a half times the length of Britain's longest (over the Humber). The Oresund is the size it is because it has to span a 10-mile stretch of water. What makes it special is the way it's been done.

Consider the engineers' assignment. On the Danish side of the water, they were asked to build a four-lane motorway that dovetailed smoothly into the existing road system, and new railway termini and tracks for both freight and passengers, in a built-up area already housing one of Europe's busiest airports.

Next, they had to carry the road and railway through nearly five miles of relatively shallow water. Then they had to lift the cars and trains over a further five miles of deep water. Finally, entering Sweden, they had to build a customs and toll point, before connecting the road lanes and rail lines with totally "foreign" networks. This positively Jovian challenge has been overcome with a series of ground-breaking (and record-breaking) innovations.

The designers compensated for the lack of available space on the Danish coast by creating some land of their own: an artificial peninsula extending 430 metres into the sea, where the passenger track from the new airport rail terminal meets the new freight line and motorway, and they head together for the first stage.

This is the bit you can't see: a two-and-a-half-mile tunnel constructed in a way that's never been attempted on such a scale before. The five tunnel bores (two for trains, two for cars and a fifth for maintenance and emergency access) were actually built on dry land in 20 sections, each weighing 50,000 tonnes. Then they were sealed, floated out to sea, sunk, connected to each other, and covered with sand and rock. Not a drill to be seen. The tubes have to be able to withstand the weight of a sunken ship or a dragging anchor and still remain rigidly in place, because cars and trains will be running within a few metres of each other. The sceptics winced at the possible dangers, but the immersed tubes have come through all their safety trials.

As vehicles emerge from the underwater section, a lattice-work roof over the motorway softens the transition between artificial lighting and sunshine, so drivers don't brake suddenly as they emerge blinking into the daylight.

For the next two-and-a-half-miles, the road and railway part company again, crossing the man-made island of Peberholm that leads to a viaduct and the bridge itself.

The island is like an immense dry stone wall. Each individual pebble and rock (gathered from elsewhere on the construction site) had to be lowered into its exact place in a jigsaw created by computer. The measurements are correct to within a millimetre or two, each checked by satellite positioning technology.

Now comes the bridge itself. Even if, for you, one bridge is much like another - and even if you're in a tearing hurry - this one will make you stop and gape. It's almost five miles long: two gradual approaches leading to a high section dominated by twin towers 204 metres high, with the international border between them.

Being the product of two such design-conscious nations, the finished product, like an IKEA flat-pack, is self-consciously clean and simple. And the panoramic view for passengers on the train is largely unobstructed.

It's definitive proof that a potentially brutalist structure of concrete, steel and tarmac, designed solely for transport, can be at once efficient and beautiful, enhancing rather than scarring its natural surroundings. There are some fierce winds in this corner of Scandinavia, yet the bridge is so secure they're expecting to have to close it to cars for a maximum of two hours a year - and to trains, not at all.

"Unlike the ferry, which in the winter suffers weeks of irregular services," says project chairman Lars Joergensen, "the train always sails."

Crossing one last viaduct to return to dry land, trains will automatically convert from Denmark's 25,000 volts to Sweden's 16,000 before heading off to Malmo. Drivers, though, must pay their dues. And this is where they do blink.

A single journey by car will cost around £19; big lorries will be charged a whopping £70. Politicians on both sides of the border are moaning that this is a disincentive to commerce, one of the main reasons the bridge was built. Joergensen, who had no control over the pricing structure, compares the cost with the $3 toll at San Francisco's Golden Gate Bridge, and is confident that market pressures will rapidly force the price downwards.

It's taken nearly seven years, and the joint consortium will need at least 30 years to pay back the £2.2bn they've had to borrow. But they've finished it on time, and four weeks from today the Queen of Denmark and the King of Sweden will set off on their respective royal trains to meet in the middle of a channel their antecedents have disputed for centuries.

After a day of celebrations, and wall-to-wall television coverage in both countries, the bridge will open to the public, and Sweden will finally become part of mainland - and mainstream - Europe.