Students in San Sebastian were asked to use the Basque language to plan a kidnapping and in Vitoria pupils were given instructions in Basque for making Molotov cocktails. Students have also been asked to write letters in Basque to prisoners jailed for terrorist activities.

The centres are subsidised by the Basque regional government and often attract separatist sympathisers.

Mari Carmen Garmendia, the Basque cultural affairs minister and spokesman for the region’s autonomous government, said the government would oversee syllabuses to guarantee that they “respect human rights and dignity”.

ETA’s recent killings of several local politicians have left its support at a new low. Criticising those teachers who seek to further the separatist cause, Fernando Savater, the respected Basque philosopher, wrote in the Spanish daily newspaper, El Pais, that “they are destroying the Basque language by turning it into an ideological choice instead of a language which is open to everyone”.

Known as euskaltegis, the language centres’ purpose is the “relearning” of Basque, a reference to the fact that centuries ago every Basque could speak the enigmatic language, believed to be more than 3,000 years old.

The use of Basque has grown apace over the past two decades with 27 per cent of the region’s 3 million inhabitants now able to speak it. Following the 1978 Spanish Constitution it has been taught in all schools, both public and private.