After four months of honeymoon, Nicolas Sarkozy starts the hard part of his presidency today when he asks more than half a million strike-happy public sector workers to give up their generous retirement packages for the national good.

Trade unions and the Socialist opposition are ready to pounce on “Super-Sarko” as he demands the sacrifices he promised to purge the French economy of its restrictive old ways.

His critics see the pension reform, and moves tomorrow to trim the civil service, as the first real test for the “Sarko Circus”, the nonstop presidential show that has dazzled and disarmed at home and abroad.

His room for manoeuvre has shrunk, with sagging growth, discontent over rising prices and a row with the EU over his failure to cut national debt. His approval ratings slipped five points to 62 per cent last week.

Mr Sarkozy’s sights are on merchant sailors and railway, utility and Paris transport employees, who have fought tooth and nail for decades to retain their so-called “special regimes” that let them retire – often early – on highly favourable terms.

Train drivers, for instance, stop work at 50 and employees of the EDF electricity giant receive €2,000 per month pension (£1,400) compared with €1,300 for private sector workers. Taxpayers are shovelling £8 billion a year into these pensions, which are enjoyed by one million retired people but financed by contributions from only 550,000 workers. Mr Sarkozy last week called this a disgrace, arguing that the logic of rewarding those in hardship jobs had vanished. Modern train drivers suffered nothing like the tough conditions of their ancestors on steam locomotives, he noted.

Hardline unionists are threatening to take to the streets, though, as they did in 1995 when strikes against pension reforms eventually brought down the first Chirac Government.

Mr Sarkozy wants the regimes to be brought into line with civil service pensions, which are still far more generous than those of the private sector.

He is confident of success because France endorsed the need for reform when he was elected last May on a platform promising a “clean break”. Opinion polls suggest that about 65 percent of the public believe that the special regimes must go.

Mr Sarkozy has been wooing the union leaders, inviting them for consulations and taking them to dinner in bistros near the Elysee Palace. All say that they will accept change only if it comes by mutual consent.

François Chereque, of the moderate CFDT federation, said: “If the Government has already made a decision and is going to try to impose it, then there will be a major conflict.” After meeting Mr Sarkozy again yesterday, Mr Chereque said that he felt that the President was not seeking a fight.

However Bernard Thibault of the militant, communist-oriented CGT union, hinted at possible disruption of the rugby World Cup when he said: “There could be sport – and not just in the rugby stadiums.”

The Socialists and other leftwing parties have organised a joint “riposte” to Mr Sarkozy’s Senate address today – at last finding a cause after months in which the “omni-President” has run rings around them.

Besides stirring the waters at home, Mr Sarkozy’s administration sent shockwaves abroad yesterday when his Foreign Minister, Bernard Kouchner, gave warning that the world had to prepare for conflict with Iran unless it curbed its nuclear ambitions.

“We have to prepare for the worst, and the worst is war,” said Mr Kouchner, one of the few prominent figures in France who supported the US-led invasion of Iraq. The remarks pleased Israel, angered Iran and stunned French allies who had grown accustomed to France positioning itself opposite the Bush Administration. Foreign ministry officials in Tehran said the remarks had damaged France’s credibility and accused Paris of abandoning the EU’s search for a diplomatic solution.

At home, some business leaders have begun to sound doubts about Mr Sarkozy’s capacity to fulfil his promises. Eyebrows were raised at the weekend when he proclaimed that France needed 3 per cent economic growth and that he would ensure it, whatever it took. Growth for 2007 is falling far below the Government’s target of 2.5 per cent, according to international agencies.

Mr Sarkozy has also fallen foul of Brussels and his European colleagues by refusing to obey the single currency rules on cutting deficits and by picking a fight with the European Central Bank for failing to drop interest rates.

After pension reform, Mr Sarkozy will face resistance from civil service unions over plans – to be announced tomorrow – to shrink Europe’s biggest public administration by replacing only one out of two retiring fonctionnaires. Within months, he will also court trouble when he starts trying to change the rigid system of employment contracts that act as one of the biggest brakes on the economy.

A blue-eyed foe

The fate of President Sarkozy’s attempt to take away public sector workers’ pension privileges lies largely in the hands of a 48-year-old former railway worker with a gravelly Paris accent, pale blue eyes and a Beatle haircut

Bernard Thibault, the Communist boss of the Confédération Generale du Travail (CGT), France’s oldest trade union, calls himself a reformer, but for France, he is the face of the old-fashioned anticapitalist workers’ cause. His mop hair and boyish face are at the front of every march

He took over in 1999 after leading a 1995 railway strike against President Chirac’s attempt at pension reform

Known as “the man who blocked France,” Mr Thibault is warning Mr Sarkozy to expect sport if he tries to foist reforms again on les cheminots (rail workers) and the other favoured workers

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