Christian MPs have warned that Sunday schools could be banned from teaching that marriage is between a man and a woman in a letter sent to the Telegraph.

The group of Conservative MPs said government proposals to subject Sunday schools and other out-of-school groups to Ofsted inspections "could have a seriously detrimental effect on the freedom of religious organisations."

The letter from Sir Gerald Howarth, Gary Streeter, David Burrowes and Fiona Bruce follows an outcry from Christian campaign groups who claim the government's counter-extremism policy is "highly alarming" as it will also target church youth groups and Sunday schools.

"The government's extremism proposals in current form pose a serious challenge to mainstream Christian views on a whole range of issues," Nola Leach, CEO of public policy charity CARE, told Christian Today.

"We are talking about the State interfering in church life and the prospect of Ofsted Inspectors sitting at the back of churches across the UK is highly alarming to say the least."

The proposals from the Department for Education came after the Prime Minister warned of a small number of Muslim groups where children have their "heads filled with poison and their hearts filled with hate."

A six-week consultation on the government's plans closed yesterday and Christian pressure groups have urged the government to reconsider after it emerged any out-of-school group providing children with "tuition, training or instruction" could face inspections.This means youth groups could face sanctions "for the expression of traditional views on matters such as marriage," according to the MPs' letter.

"This would be an intolerable but very real possibility given the clear desire of the Department for Education to investigate what it calls 'prohibitive activities', such as 'undesirable teaching... which undermines or is incompatible with fundamental British values'. This could challenge established Christian teaching," wrote the MPs.

"Threats to British values originate overwhelmingly from certain strains of Islam. It is at least disproportionate, if not absurd, to impose intrusive burdens on all other religious groups under the pretence that attempts at radicalisation could be discovered in any organisation."

The MPs join several Christian groups who have expressed their dismay at the plans.

The Christian Institute said the plans, if not curtailed, represent an "unprecedented attack on freedom of religion in our country".

However the government insisted it was not proposing to regulate Sunday schools as they only taught children "for a short period every week".

"We are looking specifically at places where children receive intensive education, to ensure that the children there are in a safe environment, which does not subject them to intolerant and hateful views," said a spokeswoman from the Department for Education.

"We recognise many out-of-school education settings do a great job in supporting children's education and our proposals are about making sure that in the small minority of cases where there are concerns raised by parents and others about issues of extremism, child cruelty or inappropriate teaching the government can take action to protect children."