Some Muslim schools "include anti-Christian chanting in assemblies" which is being ignored by the schools watchdog, it has been claimed.
The Christian Institute has said Ofsted and the Department for Education are not taking proper action to deal with some Islamic schools accused of bullying Christian pupils.
In a letter to Nicky Morgan, the Education Secretary and a Christian, the Institute's director Colin Hart said "inadequate attention" had been given to the way Christian pupils were being taught.
He said it was "striking" that several official reports outlining examples of Muslim "intolerance" to Christians had gone ignored by Ofsted.
"You will see they include anti-Christian chanting in assemblies, GCSE pupils who opted to learn Christianity being left to teach themselves, and Christians being called 'ignorant' or 'liars' by teachers," he wrote.
It was "disturbing" that there had been a "complete lack of any specific plans of action to tackle anti-Christian intolerance", he added.
Mr Hart also accused the school's inspector of being part of the problem and said its methods for detecting bullying against Christians were "clearly inadequate".
"Ofsted may be part of the problem", he said.
"Whilst guidance from your department says there is no obligation on schools to promote gay marriage, there have been Ofsted inspections in several parts of the country where Ofsted inspectors have implied, via questioning of pupils, that they ought to accept gay marriage there have been Ofsted inspections in several parts of the country where Ofsted inspectors have implied, via questioning of pupils, that they ought to accept gay marriage.
"We are aware of several cases of this approach being taken with primary age pupils, some as young as six."
In a statement, Ofsted said: "Ofsted deplores bullying in all its forms. We expect schools to promote British values including mutual respect and tolerance of those with different faiths and beliefs.
"However, it is nonsense to suggest that an Ofsted inspector would expect a school to have taught six year old pupils about same sex marriage."
A DfE spokesman said: "Bullying of any kind is unacceptable and all schools should take an active role in tackling it. We expect all schools to actively promote fundamental British values, which include mutual respect and tolerance of those with different faiths and beliefs.
"Ofsted plays a key part in ensuring this takes place.
"Heads, teachers, governors and trustees are working tirelessly to resolve deep seated issues at these Birmingham schools which developed over a long period of time."