All private schools will be forced by law to teach English for the first time under new regulations being proposed by the Government.
The rule, due to come into force in September, is aimed at religious schools. "This regulation would apply mainly to Jewish and Muslim schools," said a Department for Education and Skills spokesperson.
The tougher rules follow Office for Standards in Education reports that a few religious schools, especially those serving Orthodox Jewish communities, were failing to provide satisfactory English tuition.
But Muslim schools in the UK said they were "baffled" that they had been targeted by the DfES, as they already give English teaching a high priority.
Idris Mears of the Association of Muslim Schools said that some Muslim schools scored highly in recent value-added league tables.
He said that even in those Muslim secondary schools that concentrated on religious studies and the Koran, the majority of teaching was done in English.
English is also the medium of most private Jewish schools. Simon Goulden, chief executive of the Association for Jewish Education, said: "We function in the mainstream of voluntary aided and independent schools which follow the national curriculum. Our independent schools would have no difficulty with this regulation."
Rabbi Abraham Pinter, principal of Yesodey Hatorah school in Hackney, London, which appeared near the top of the value-added league tables, said English was taught in all Jewish schools "to some level". He said the new rules could become an issue for some if they meant schools had to teach English to GCSE. But some ultra-Orthodox Jewish schools teaching mainly in Hebrew or Yiddish have been criticised by inspectors for unsatisfactory secular curricula.
The proposed new regulations state: "Where the principal language of instruction is a language other than English, (schools must provide) lessons in written and spoken English."
The exceptions are schools for temporary residents who follow the curriculum of other countries, such as those serving the Swedish and German communities in London.
The consultation on the new law closes at the end of May.