UK school inspectors say there is a "culture of fear and intimidation'' at several British schools investigated over allegations of a plot by Muslim "extremists" to run them along strict Islamic lines.

The Ofsted education watchdog said on Monday that five of 21 schools it inspected in the city of Birmingham failed to protect pupils from the risk of "radicalisation and extremism".

A summary of the report by the chief schools inspector, Michael Wilshaw, stated: "A culture of fear and intimidation has developed. Some headteachers reported that there has been an organised campaign to target certain schools in Birmingham in order to alter their character and ethos.

"Birmingham City Council has failed to support a number of schools in their efforts to keep pupils safe from the potential risks of radicalisation and extremism.

"It has not dealt adequately with complaints from headteachers about the conduct of governors."

The inspectors recommended that authorities should "further investigate whether there has been organised infiltration and manipulation of governing bodies".

The inspections were spurred by an anonymous letter alleging a plot called "Operation Trojan Horse" by radical Muslims to infiltrate Birmingham schools.

Authorities believe the letter was a hoax, but the alleged plot inflamed tensions in Britain's second-largest city.

The Park View Educational Trust, which runs several of the schools, insisted they "do not tolerate or promote extremism".

David Cameron, the British prime minister, earlier on Monday promised a "robust response" to allegations of "Islamic extremism" in schools.

In a statement he said: "Protecting our children is one of the first duties of government and that is why the issue of alleged Islamist extremism in Birmingham schools demands a robust response."

Source: Al Jazeera and agencies