President François Hollande faces an agonising decision in the new year – not about France’s many problems but about one of its great successes, the high-speed train.

As Britain finally prepares to climb aboard the railway revolution first popularised by France 30 years ago, the train à grande vitesse (TGV) is heading full tilt towards controversy in its homeland.

Mr Hollande must decide by mid-January whether to give the go-ahead for a new high-speed line which would cut journey times to his own political fiefdom in a thinly populated region of south-western France.

The President has pledged privately to local politicians that the €1.8bn Poitiers-Limoges line will be built. The French state watchdog has concluded that the project is a high-speed branch line and economically absurd. The government must make a decision by 12 January.

Argument is already raging about an iconoclastic report from the Court of Auditors – the country’s financial watchdog – which suggests that all existing high-speed lines in France are losing money if the debt on their building costs is taken into consideration.

Passenger numbers are falling; the price of tickets is rising steeply; the TGV system faces new competition from a decision to create a nationwide bus network. At the same time, opposition is growing from wine producers, farmers and militant environmentalists to well-advanced plans for two further high-speed lines in the south of the country.

The decisions taken by the French government early next year will be closely watched in Britain. Opponents of the HS2 high-speed line to the Midlands and the North will seize on any sign that Paris is dismantling its ambitious plans to double, or even triple, its 2,000km-network of high speed lines by the middle of the century.

Defenders of HS2 will, however, point to the many successes of high-speed rail travel in France and beyond.

More than two billion passengers have been carried on high-speed lines in France since September 1981, without a single death or serious injury. The express train, once regarded as a defunct form of transport, is now seen as a viable, safe, green alternative to the plane on five continents. 

High-speed lines are in use in China and Russia and planned in the US, Saudi Arabia, Argentina and Morocco. A Europe-wide network is taking shape in France, Germany, Benelux, Spain and Italy.

Although Japan pioneered high-speed railway lines from the 1960s, it was France that captured the world’s imagination in 1981 by developing a robust, 150mph – and now 200mph – train capable of travelling very fast on special lines, and quite fast on conventional tracks. Unlike the Japanese bullet trains, TGVs could use existing big-city stations and reach a multitude of destinations.

Four new French fast lines – or extensions to existing lines – are currently being built. They will shorten journey times to Bordeaux, Rennes, Strasbourg and Montpellier by 2017. There are plans – not yet finalised – for new lines by 2030 from Bordeaux to Toulouse, Bordeaux to the Spanish border and the first section of a line from Lyon to Milan (via an Alpine tunnel).  To this may be added next month President Hollande’s pet scheme for a single track TGV branch line to Limoges, leaving the new Paris-Bordeaux line at Poitiers.

Can a struggling country afford so many new railway lines? The recent Court of Auditors’ report was intended as a danger signal.

The court suggested that the French network had already been allowed to sprawl too far and too fast. Of six high-speed lines studied, none had reached its promised target for passengers carried, the court said.

Although the train services themselves were profitable, no French high-speed line – not even the pioneer line from Paris to Lyon – could be said to be covering its total capital investment and maintenance costs. Worse, the court said, the claimed wider economic benefits of high-speed rail had not fully materialised.  

Some of these conclusions are disputed by the government, by the state railway company, the SNCF, and by trade unions.

Bernard Aubin, head of the First rail-workers’ union and a columnist and blogger on railway issues, told The Independent: “The report is a disgrace. Does the Court of Auditors ask whether new motorways are profitable? Or new airports?”

“All high-speed rail services in France are profitable. It is only when you include the whole capital cost – the money invested not just by the railways but also national and local government and Brussels – that you can argue that they make a loss.

“The lines were built as a public investment in the economy of the country. It is misleading to charge debt on all the capital cost to the operational bottom line.”

The auditor’s report does make one important argument, however. The infrastructure arm of French railways – the equivalent of Network Rail – has a mountain of €35bn of accumulated debt. Most of this comes from its share of building the new high-speed lines.

To service the debt, it has been forced to increase steeply its “toll fees” to the SNCF. Ticket prices have soared. Passenger numbers have fallen. Much of the non-TGV rail network has fallen into disrepair.

Even Barbara Dalibard, head of passenger services at the SNCF, accepts this point. “After 30 years of success, the more we extend the high-speed lines, the less profitable they are,” she said. “What the Court of Auditors says is that we have reached the limits of expansion of the system. We agree with that.”

There are lessons here for Britain – but not necessarily simple ones. HS2, linking London with the vast Midlands and northern conurbations, should, in theory, be ideal commercial territory for high-speed rail travel.

The core French network – the lines from Paris to Lyon and Marseille, to Lille, Belgium and the Channel Tunnel – is broadly speaking a success. As the French Court of Auditors points out, it is the long lines through thinly populated country to Alsace, Brittany and the south-west, which have stretched the TGV economic model to the breaking point.

Can a struggling government really deliver the President’s promise next month to build what is already being called the “ligne François Hollande” into the empty south-west of France?