Their action has gained strong support from teenage pupils, who have joined in, dominating two big demonstrations in Paris last week that led to many school closures.

French teachers’ unions gave notice of “large-scale mobilisation” last year and have so far staged strikes and demonstrations in November, January and March.

They have been campaigning since shortly after the general election in June, which gave a comfortable majority to President Nicolas Sarkozy’s conservative UMP party.

Teachers were angered by a government decision to replace only one in two retiring civil servants in a category that includes teachers and other school staff who are state employees, which they say will cost 11,200 educational jobs.

In a joint statement, the five principal teaching union federations condemned “this new stage in decline of the public education service on a scale never previously attained”, which meant “unacceptable deterioration for pupils and young people as well as for staff”.

In recent weeks, students from lycees, France’s upper secondary schools, have joined the protests.

UNI and Fidl, the two main organisations representing teachers of the pupils, supported demonstrations this week and a protest in Paris last week, which attracted up to 40,000 students and teachers.

Some lycees have been closed and blocked by young protesters, often supported by their teachers.

The lycees’ grievances include the size of classes, which will become even larger from September, when the policy of staff non-replacement comes into effect, and because of plans to cut vocational baccalaureat courses from four years to three. The pupils say they are worried for their futures, fearing qualifications that are devalued, job insecurity and unemployment.

The education ministry has justified the cuts in posts by saying the number of lycee students has been declining over the past four years.

Xavier Darcos, the education minister, told Le Figaro last week that the teachers’ action was “disproportionate”. Only about 8,500 teaching jobs would be cut, and about half the hours lost would be made up by voluntary overtime. “So there will only be (the equivalent of) 3,500 posts not renewed, out of nearly a million teachers.”

Mr Darcos met representatives of UNI and Fidl last Friday, but only to talk about curriculum reforms, He had meetings scheduled with teachers’ unions this week.

The teenagers’ representatives have called for supporters to continue the protests because “big disagreement on resources” remained.

Teachers’ unions have fixed further days of action in May.