The Prevent strategy is widely felt to be inhibiting free speech in schools and encouraging teachers to avoid “toxic” issues of extremism, according to a government watchdog.

David Anderson QC, the independent reviewer of terrorism legislation, said the way the policy has been implemented left teachers feeling “vulnerable” and reluctant to confront radicalisation.

In some instances, the policy even stops some Muslim parents from talking to their own children in case inaccurate versions of their views are repeated in school, he said.

Giving evidence alongside Anderson to parliament’s joint select committee on human rights, his predecessor, Lord Carlile of Berriew QC, called for more checks – on child protection grounds – for after-school madrassas that teach Islam.

Both lawyers said they believed the anti-extremism Prevent strategy did good work and that the government should reveal more about its methods and successes to broaden public acceptance.

Anderson, who has recently been talking to Muslim families in Leicester, Dewsbury, Bolton and other cities, said most Muslims entirely approve of “British values”. But he had heard complaints about Prevent mainly on the grounds that it stifles free expression, he told MPs and peers.

“One lady [teacher] in the north-west said that Isis comes up quite often and she used to use it as an opportunity for a discussion: why are they using violence, what about other ways, what about Martin Luther King, what about Mahatma Gandhi, someone mentioned the IRA – are they the same as Isis? They would have had a discussion.

“The toxic views would come out and they would either be blunted or neutralised, or at least [pupils] would be given something to think about. Now, she said, you choke off the discussion because teachers are watching their backs and don’t want to be reported.”

Another complaint Anderson cited was from Muslim parents “who don’t like talking about these things in the home because if the subject comes up and the child goes into school the next day and perhaps gives an inaccurate or colourful account of what [was said], then some half-trained teacher may make a Prevent referral and the whole family would be in trouble”.

Other teachers feel vulnerable, Anderson added: “I met one who was at the time suspended from school … A fellow teacher, who perhaps did not like her very much, had complained about her and indicated that she intended to go to Syria that summer.

“The teacher told me that she had said she was going to attend a fundraising dinner for Syria, but a Prevent referral was made and she was suspended. She was the only Muslim teacher in a well-to-do white school in an area south of Manchester.

“So all these things make people feel inhibited. You can say it’s a breach of their human rights but a more powerful matter to me is why aren’t these matters being ventilated and talked about in a controlled manner in a space where ideas can be challenged.”

Carlile told the committee that he would like to see more checks on after-school madrassas where children are taught to recite the Qur’an, often for two hours an evening, five nights a week.

“I believe there are child protection issues that need to be addressed,” he said. “These are large numbers of hours after school. In my view some type of light-touch regulation or inspection would be justified.”

Both Anderson and Carlile said they expected the government’s forthcoming counter-extremism bill – which is supposed to give powers to close down premises and ban groups – was likely to be heavily diluted when it eventually comes to parliament.