A law lifting the cap on the number of such schools in the state goes into effect in the new year. It would allow California to grant charters to about 100 new schools a year, in addition to the 150 already in place.

The numbers are still small compared to the 8,000 traditional state schools in California. But the state has a rich tradition as a US trend-setter, and the new schools will help make charters impossible to ignore on the national scene.

The first charter schools were established in Minnesota in 1991 and have since been seized on by President Bill Clinton and reform-minded conservatives alike as an innovation almost everyone can agree on.

Charter schools operate outside the control of local school boards or districts, though their charters are periodically reviewed and may be revoked if they do not meet certain standards.

Supporters say that creating schools independent of large education bureaucracies is a means to reinvigorate teachers, pupils and parents. Their small size gives them great flexibility. By creating competition, they also spur regular schools to reform, it is argued.

In part because of the lack of national and even state-wide standard tests in the US, however, hard data on their accomplishments is scarce. Critics contend charter schools are an excuse for elitism and economic and racial segregation, and even a threat to the principle of a state education for all.

California’s bold new step comes, paradoxically, at a time when it is staging a major retreat from earlier educational experiments. “Whole-language” reading instruction, and “new maths” courses have been rejected in favour of a “back-to-basics” curriculum stressing phonics and old-fashioned maths skills.

There are currently only about 1,000 charter schools in the US, but 279 new schools were added in the 1997-98 school year alone. President Clinton has called for 3,000 of them by 2000. They are equally popular, for the most part, with those on the Right and the Left.

Charter schools have been championed in the conservative Western state of Arizona, which leads the country with 250.

Rosa Parks, the celebrated civil rights campaigner who challenged racist segregation in the American South in the 1960s, has led moves to open a charter school in Detroit, Michigan.

While some black and Hispanic leaders from urban areas have voiced doubts, the schools also seem popular with parents. A Department of Education study found 70 per cent of charter schools are over-subscribed.

The drive to establish more charter schools was led by Silicon Valley multi-millionaire Reed Hastings, who has used some of his estimated pound;530 million fortune to campaign for them.

California’s new Democrat Governor, Gray Davis, who was elected last month carrying the flag for education reform, has appointed Mr Hastings to his education task force.