Tony Blair does it with Nicolas Sarkozy, but Angela Merkel prefers not to do it with Mr Sarkozy. His new Government has announced that it will be banned in schools.

“It” is the use of the familiar second person singular – “tu” in French, rather than the more formal “vous”.

French habits have greatly relaxed since the days when only the lower classes and hard leftists used “tu” with people who were not intimates, but the verbal code remains a mine-field. Life was easier, some say, when everyone obeyed the rules. Now, a movement is afoot to restore them.

President Sarkozy’s new Government has stirred up the French education world and pleased traditionalists by ordering all pupils to say “vous” to their teachers and advising les professeursto apply the respectful “vous” to their older school students.

The head of state, who has brought a cool touch to the Elysée Palace, committed a gaffe of his own when he visited the German Chancellor in Berlin on his first day of office last week. Mrs Merkel dropped German formality enough to call him “Lieber (Dear) Nicolas” but stuck to the formal “sie” not the familiar “du”. Mr Sarkozy’s matey reply jarred on old-fashioned ears. “Chère Angela . . . J’ai confi-ance en toi.” (instead of “en vous”) Libération joked that Franco-German harmony was still lacking. “They are going to have to start by agreeing whether they use ‘tu’ or ‘vous’,” it said.

Mr Blair and Mr Sarkozy, who consider themselves friends, always tutoie one another. Gordon Brown’s lack of French – and Mr Sarkozy’s poor English – will remove the problem because they will use interpreters.

President Chirac used “vous” with Mr Blair in public. When Mr Blair, who picked up his French working as a Paris barman in 1975, tutoied Mr Chirac, who is 20 years his senior, officials put his overfamiliarity down to ignorance.

There is a contradiction in Mr Sarkozy’s modern use of “tu”. The right-wing reformer won office with a back-to-tradition campaign that blamed the 1968 student revolt for breaking down French society. “I will liquidate the legacy of May 1968, with its abandonment of moral codes,” he promised.

The schools order is part of Mr Sarkozy’s campaign to reimpose respect and civility across French society. Since the 1960s generation threw off formality, some teachers have let pupils tutoie them and most tutoie their pupils.

Xavier Darcos, the new Education Minister, said: “It is indispensible that children vouvoient their teachers and preferable that teachers do not use ‘tu’ with lycée pupils, so that everyone is in their right place.”

Left-wing teachers’ unions accused Mr Darcos yesterday of exaggerating the problem because few pupils used the familiar second person to address their teachers. The Minister said that he did not mean to damage relations between teacher and pupil but “there must be a necessary distance, defined by le vouvoiement”.

The French adoption of “tu” has not been welcomed universally. Le Figaro complained on Monday that le vouvoiement had been “laid low by rampant tutoiement spreading from the business world imitating the Anglo-Saxons and now invading private life”.

Some French couples in high bourgeois and aristocratic families still vouvoie one another and their children. Even the trades unions say that the systematic “tu”, laid down in some company rules, is a scurrilous plot against the workers, by which the bosses are robbing unions of their dignity and authority.

“It is difficult to call each other “tu” when you are defending staff who have been sacked,” one union official told Tribune newspaper this week.

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