More than 2,000 invited guests and representatives of the world's media will swoop on Folkestone and Calais to watch the heads of state try out the tunnel. Yet the elaborate celebrations have a hollow ring: lengthy testing of trains has meant the tunnel is still not open for business. A limited inter-capital Eurostar foot passenger service will start in July the first Le Shuttle car services are due to start in October.

The maiden trip in the first fully-laden passenger train will be made by the 800-strong British contingent who leave Waterloo International terminal this morning. Among them will be Prime Minister John Major and his predecessor, Baroness Thatcher, who in the 1980s pushed the project through Westminster.

President Mitterrand arrives in Calais on board another Eurostar train bearing the French great and the good. After unveiling a plaque, he and the Queen return to Folkestone in her Rolls Royce on a chrome shuttle train.

The French train, of course, will travel at its top speed of 300kph (186mph) on the French network. Its British counterpart will travel at no more than 60mph for the first 12 miles out of London and no faster than 100mph anywhere else.

As with all major engineering achievements there are plenty of facts and figures to throw around - the longest undersea tunnel in the world (38 kilometres under the Channel) the second longest rail tunnel (50 kilometres) 42 million hours of work 11 tunnel boring machines (TBMs) weighing 140,000 tonnes each 20 million cubic metres of earth and rubble (equivalent to 14 pyramids of Cheops) seven days of strikes four million tonnes of concrete FFr100 billion involving 220 banks 600 carriages and locomotives a year and a half of delay 10 deaths 15 million expected passengers and six anti-fox barriers.

At the information centre at Coquelles an ammonite fossil 40 centimetres across is displayed as a trophy. It was found on 23 June 1991, 8,783 metres from the French coast,102 metres below sea-level. It dates the geological stratum at 95 million years. That is the date of the formation of this particular chalk marl, which is a stable combination of limestone and clay, easy to bore through and proof against flooding, yet solid and impermeable.

Luckily, this layer of rock runs uninterrupted at a thickness of 30 metres from Shakespeare Cliff at Folkestone to Le Puit de Sangatte near Calais. The Eurotunnel engineers say that without this geological peculiarity, it would have been impossible to dig the tunnel. Above it is white chalk and grey chalk: solid, but porous as a sponge and far more difficult to deal with. The tunnellers avoided this layer as much as possible.

Underneath is Gault clay. 'It is just like the stuff they use for pottery. We were really worried that the machines, the famous TBMs, were going to hit that stratum, but it never happened,' says a spokesman for the engineers. 'Previous tunnel attempts have fallen foul of the very basic geology of Europe.'

Andre Benard, the 63-year-old co-chairman of Eurotunnel, is the man who put the whole thing into practice. 'Traditionally, deep down, the British look to the open sea,' he says. 'Yet now, a fixed, visible link to the Continent is forcing them to turn their attention to Europe.' He feels that this is the fundamental point at a time when people all over Europe are beginning to question the Maastricht Treaty and their common future. 'The French have always been fascinated by theories from the Eastern Bloc. We are the last conservatoire of Marxist theory, a regime where the citizens serve the state and the state serves itself. The tunnel acts as a liberalising counter-balance to this philosophy.' A hundred billion francs for a liberalising corridor 50 kilometres long. But is that justification enough? 'Yes, undeniably,' says Benard.

It took 27 attempts to get it right. Tunnels under the sea, tunnels under the sea bed, bridges on pontoons or suspended from fixed balloons, barrages - to harness water and wave power - the Channel has seen it all.

Aime Thome de Gamond (1807-1876) devoted his life to the tunnel. Albert Mathieu, in 1802, convinced Napoleon Bonaparte of the benefits of a tunnel. The English have always been less enthusiastic for mainly military reasons, but also to protect that insularity that has forged their history.

It wasn't until 1955 that the British Ministry of Defence was prepared to accept the idea of a tunnel. Then in 1964, the Pompidou government signed a joint declaration in favour of building one. The work didn't start until 1973 and stopped two years later. In 1981, came Thatcher and Mitterrand, both freshly elected. 'Thatcher wanted to make a concrete pro-European gesture,' says Benard. 'But she also wanted to show what the private sector could do. Mitterrand was also pro-European and he also had a real taste for grand public works. They met, they came to an agreement. There would be a tunnel. And it would be funded by the private sector.'

Construction firms and banks immediately dredged up the old tunnel projects, and got to work on them. By the deadline of 31 October 1985, five projects had been proposed. Euroroute (a submerged rail tunnel and a tunnel joining two artificial islands themselves joined to the two coasts by bridges) the Europont project (a suspension bridge and a rail tunnel), Transmanche Express (two underground rail tunnels and two underground road tunnels the Van de Put ten project (a dyke, an underground tunnel and a dyke for trains and vehicles). And finally the France-L Manche- Channel Tunnel Group project (two underground rail tunnels, with rail shuttles to carry vehicles, plus a service tunnel). At that point, the battle became purely ideological.

For Thatcher, the only viable option was a cross-channel motorway. The car is the perfect individualist means of transport and thus fitted with her concept of personal freedom. The shuttle-train taking charge of vehicles seemed like the very image of state intervention. Eighty per cent of British people agreed with her. 'I shall be the first to drive through the tunnel at the wheel of my car,' she said. For France, blessed with a highly developed train culture and a highly developed culture of collectivity, the car-train seemed the most obvious and most economic option. For a start, there would be no need for a very sophisticated ventilation system as there would be no car fumes to release from the tunnel.

It was a real battle of wills and, in the end, Thatcher was forced to give way. At Lille on 20 January 1986, Francois Mitterrand and Margaret Thatcher announced that Eurotunnel-Transmanche had won the contract.

So it was the railway that won the day. But Margaret Thatcher gained a small compensation and a significant victory. The compensation: in the tunnel the trains would run on the left like most other trains in the world. And this would also apply to the maintenance and security vehicles in the service tunnels. The victory: there would be no public funding. Not even a state guarantee to the bankers. Just agreements with British Rail and SNCF (the French rail system) guaranteeing that they would use the tunnel. 'It's a superb opportunity for the private sector,' said Thatcher. It allowed her to show that, like the Suez Canal in the 19th century, private enterprise could fund the kind of massive engineering projects that, little by little we had come to believe were the sole reserve of the state sector.

At Lille town hall, the winning consortium was overjoyed. The French companies included Bouyges, Dumez, Societe Auxilliere d'Enterprises, Societe Generale d'Enterprises, SPIE/Batignolles, and the British Balfour Beatty, Costain, Tarmac, Taylor Woodrow and Wimpey. And the banks: Suez, Credit Lyonnais, Banque Nationale de Paris. They predicted the tunnel would cost 47 billion francs. That left just the chalk marl to deal with. On a deserted beach on the north coast of France is a huge concrete tank buried in the ground. It is surrounded by a double ring of fences, the second one electrified. Now the Puit de Sangatte provides air to the tunnel, but it was here that the French started digging.

The first TBM - capable of extracting 2,000 tonnes of earth and water in an hour - was christened Brigitte after the wife of the site director, Pierre Matheron.

At the beginning, nothing worked on the French side. The machinery was too sophisticated and untried. Brigitte's drive mechanism broke down. In three months, the French had dug just 50 metres of tunnel. Meanwhile the British, who began work on 15 December 1987 where the previous attempt had left off in 1974, were advancing by 100 metres a day. Matheron and Benard were furious.

Yet slowly the chalk marl came to the surface and this created a new problem - where to put all the rubble. The British decided to dump it in the sea to reclaim land off Folkestone. The French constructed a huge lagoon between the hills and the sea and pumped in slurry made from the mixture of water, mud and chalk marl.

In June 1990, the first British boring machine entered French territory. In the following November, the two machines digging the service tunnel stopped within 100 metres of each other and the British sent through a test probe. They were out of alignment by 30cm horizontally and 8cm vertically. And, said Eurotunnel, 'We had underestimated the length of the tunnel by two centimetres.' Once again, the British press was on form: 'We can smell the whiff of garlic coming through the tunnel.'

It took a month to dig the final 100 metres. And, at last, on 1 December 1990, at 12.12pm, French tunnel worker Philippe Cozette shook hands with the British tunnel worker Robert Graham Fagg. They exchanged flags and cigarettes (unlike the French, the British weren't allowed to smoke in the tunnel). Amid emotional scenes, the human link was made.

By contrast, everything was going badly wrong on the surface. 'There were bad signs as early as September 1987,' says Benard. 'All we had to do was reinforce the wall holding up Shakespeare Cliff at Folkestone.' The projected cost in July was pounds 28 million. 'In September, the construction companies sent in their bill: pounds 40 million. At the end of September, it became pounds 50 million. Slowly but surely it rose to pounds 70 million.' They were over-budget for the first time, but it would not be the last.

But in the British parliament, the pro-tunnel argument had finally won the day. A study of the figures is revealing there was a progressive growth in the support of MPs for the tunnel (a very powerful Eurotunnel lobby played its part in this).

In February 1986, the House of Commons passed the Channel Tunnel legislation by 268 votes to 107. In June 1986, the second reading was passed by 282 to 87 and in February of the next year, it became law by 309 votes to 44. In France, the bill was adopted unanimously by the National Assembly and the Senate. On 29 July 1987, Francois Mitterrand and Margaret Thatcher signed the Treaty of Canterbury.

Why was the Chunnel so expensive? There are five main reasons. The divisive nature of Europe the greed of the construction companies the demands of the banks, red tape and the scale of the project.

Examination of diverse European standards throws light on the other four factors. Take one example: in the service tunnel, there are two types of fire extinguisher - one English, the other French two types of plug, and signs in both languages everywhere. With the rolling stock, it's even worse. This duplication has sent bills soaring.

The French government moved swiftly to announce the creation of a northern TGV line, starting at the tunnel - a logical move, but expensive. In theory, these TGVs should be able to run everywhere in France, Belgium and through the tunnel to England. 'Generally the price of a classic TGV with 18 carriages and two engines is around FFr 160 million,' according to Yves Machefert-Tassin, rail consultant to Eurotunnel. The Eurostar passenger trains have to be adapted to English lines at 700 volts, and to 3,000 volts for Belgian lines. The cost of adaption? FFr210 million - over 30 per cent more expensive than a conventional TGV.

Then there are the construction companies who have controlled the project since the beginning. These are the men who created Trans Manche Link (TML), the firm which built the tunnel, and the administrative parent company Eurotunnel. They have a 65-year concession to exploit the Chunnel. They also negotiated their own contracts. 'When I arrived, these contracts were signed they were rather one-sided and they have never been renegotiated correctly,' says Benard. Neither he nor Sir Alastair Morton, his British counterpart who is considered to be a tough businessman, were able to change a line. For example, TML ordered the rolling stock on behalf of Eurotunnel, and took a cut. More expensive rolling stock meant more revenue for TML.

However, because neither company had enough money to see the project through, they had to borrow from banks. The financiers used their powerful lever on the constructors. According to Benard, the agreements were extremely rigid. The 220 financial institutions supporting Eurotunnel lent around FFr60 billion ( pounds 7 billion) which has brought them FFr30 billion in interest and commission. There were further miscalculations. All the budget estimates for the cost of the rolling stock were made at the same time as the initial technical plans - in 1974. At that time, the maximum size of lorries was 4 metres high, 2.5 metres wide and 14 metres long with a maximum tonnage of 40 tonnes. Two decades on, those specifications have gone up to 4.2 metres high, 18.5 metres long, and they now weigh up to 44 tonnes. The whole transport system had to be revised.

The security measures all had to be tightened up. 'The Inter-Governmental Commission set up to monitor the project in the name of both governments gradually added more and more regulations,' says a Eurotunnel official who did not want to be named. 'The French did not want to be seen as less safety-conscious than the Brits and vice-versa. So they continued adding safety measures, all appropriate but often imposed at the last minute which contributed to delays and over-spending.' There was thus an overspending of 70 per cent on digging the tunnel, 55 per cent on fixtures and fittings, and 230 per cent on rolling stock. And a serious delay. The tunnel was due to open in spring 1993, then it was put back to late summer, then to December, and finally to 7 March 1994.

Two weeks ago, Eurotunnel admitted that it will not be able to launch the full 'turn up and go' car shuttle service until next summer - with the first fare-paying services not starting until October 1995. But if today's inauguration marks the fulfilment of a century's-old dream come true, it also signals the beginning of the new, most critical phase for the tunnel.

After a last-minute scramble to get the first operating certificate from the Inter-Governmental Commission earlier this week, it looked as though the project could be open for business next Monday with the launch of limited 'invitation-only' freight shuttle services.

The tunnel is built and the two heads of state have given it their blessing as a symbolic step forward in the development of Europe. Now it has to prove itself not only as a transport project that works safely and reliably, but one that passengers an d businesses want to use, and - as far as the banks are concerned - one that returns the mountains of debt. Traffic flows still have to be turned into cash flows. Yet this inauguration is a key date. It is not certain that Britain will cease to be an island neither is it clear that that is what the people on both sides of the Channel really want. However, the completion of these three tunnels through the chalk marl is the concrete embodiment of 192 years of dreams.

In 1803, when Napoleon Bonaparte explained the first tunnel scheme to the leader of the Whig Party, the Englishman cried enthusiastically: 'It is one of those grand projects we can do together!' Finally, it is done.