Assurances that Ofsted inspectors will not inspect Sunday schools have failed to satisfy Christian MPs who today expressed outrage at the government's "astonishing" and "worrying" proposals.

Ahead of a well-attended and lively debate this morning, David Cameron wrote to Tory MP Gerald Howarth saying the government "is not proposing to regulate institutions teaching children for a short period every week such as Sunday schools or the Scouts".

However MPs told Christian Today they were deeply dissatisfied with the response. Catholic MP Sir Edward Leigh, who led the animated debate, said he anticipated the government's angle but he remained unhappy.

"It is the cumulative effect we're worried about," he said after the debate, reflecting concerns raised by a number of MPs from all parties. They pointed out that children often not only attend Sunday schools but also church services and community groups which would take them over the six to eight hour threshold set by the government, after which they would be eligible for inspection.

"The only response I will satisfied with is these proposals are completely abandoned," Conservative MP Fiona Bruce told Christian Today.

She said the move could well be illegal and a breach of the European convention of human rights (ECHR) which "excludes any discretion on the part of the state to determine whether religious beliefs or the means used to express such beliefs are legitimate".

Labour's faith envoy Stephen Timms joined at least 20 other MPs from his and other parties in condemning the policy as "deeply unattractive".

"This measure would make Ofsted the state regulator of religion," he said during the debate.

"It is simply wrong to suggest that someone who is deeply religious should as a consequence be regarded by the authorities as suspect," he told Christian Today afterwards.

A number of MPs referred to "huge concern" expressed by constituents and Christian groups who had written to them. CARE, a Christian policy organisation that has campaigned against the regulations, said they were "more Big Brother than big society".

"If the State is granted power in the form of Ofsted inspection of church activities we are talking about a radical and alarming altering of the established relationship between church and state," said CARE's CEO Nola Leach.

"How can it be right to make Ofsted inspectors the new custodians of British values and able to decide whether or not some aspect of Christian teaching is right or wrong? This has serious implications for free speech."

Today's debate comes after Sir Michael Wilshaw, head of school inspection body Ofsted, said all religious groups training children for more than six to eight hours per week would be forced to register and would be inspected if concerns were raised about their teaching.

Education minister Nick Gibb today confirmed that "if an institution is teaching children intensively they will need to register".