But three months after its victory, in open competition with four other firms, all French, there is a hitch. Political and professional opposition in France means the result could be ignored; the final decision is said to rest with President Jacques Chirac. The Millau stretch of the motorway was always going to be controversial.
The town is at the conjunction of two valleys. The descent from the north and the ascent to the south afford views of mountains, forests and pastures, and frustration for motorists seething behind lorries.The beauty of the surroundings meant any road bigger or straighter than the present one would be contested on environmental grounds, however much Millau might be suffocated by traffic. It is not the decision to complete the motorway, nor the route chosen, that is causing most difficulties, but that the contract has gone to a foreign firm. References to aesthetic "shortcomings" of the design spill out occasionally but objectors are working almost entirely through the corridors of power. The contract has still not been finalised.
From the Millau office that is the hub of the A75 planning, Georges Gillet, head of operations, insists the "sensitivity" of the current stage of the contract is not a French-British problem, but, though he declines to say directly, a "French-French"
problem, and one France is likely to face increasingly as European markets become more open and competitive.
The brief, he said, was kept as wide as possible and was more than a simple design competition. The shortlisted firms had to decide first which of five proposed routes and bridge types they found most appropriate and design accordingly.The Foster design is for a 2,500m viaduct west of Millau; its tallest supports will be higher than the Eiffel Tower. A model of the light and elegant design resides in a glass case in the foyer of the office in Millau.
The official announcement in July said Foster had won on a combination of design, technical and environmental grounds and with an estimated cost "significantly" (said to be 30 per cent) less than most of the other submissions. Mr Gillet lauded the design as little short of an architectural and engineering marvel.
However, given the influential opposition that has subsequently made itself felt, how was the British firm able to win?
The deliberations of the jury are secret, and all that is known is that the decision was made by "an absolute majority" (ie it was not unanimous). But one factor may have been decisive.
In the competition to design the Millau viaduct, all the rules about public-works tendering, including appointment of international advisers and a mixed jury, appear to have been observed. As scandals in Paris and elsewhere show, this does not always happen, and it may have owed something to the fact that Mr Gillet, as design and works head for the whole of the A75's southern sector, was, unusually, given authority which passed across four regional boundaries. The pressure that could be exerted by politicians of any one region was thereby diminished.
Foster and Partners are no strangers to controversy in France. They recently faced down opposition to complete the Carre d'Art in Nimes, a big complex in a conservation area of the city centre. The scheme is acknowledged to have revived and enhanced an erstwhile forlorn part of the city.
The Millau viaduct, however, is a project of a different order. It has national, not just local or regional, significance. The head of the motorway project and local officials hope it will become an attraction in its own right and not just a six-lane traffic conveyor.
From officials at the Department of Road Transport, word is that the contract is being drawn up "as a matter of priority". Pressed about mooted difficulties, they concede there is opposition - "from some French architects" - but insist that the competition result stands.
In London, Foster and Partners acknowledge the delicacy of the project at its current stage but clearly hope the opposition will be overcome. Work on the Great Millau Viaduct is due to start in 1998, for completion, with the A75, in 2001.