Legal action is being threatened against the Government unless it allows pupils to exempt themselves from religious assemblies.
The National Secular Society (NSS) said it would consider taking the Government to judicial review if it did not follow the advice of a cross- party human rights committee and make acts of collective worship in schools optional.
State schools are required to hold a daily act of "broadly Christian" worship. A study by Ofsted in 2005 found that this took place in 98 per cent of primary schools, but only 17 per cent of secondaries.
The committee's report earlier this month recommended making religious assemblies and RE optional.
"Provisions that fail to guarantee a child of sufficient maturity, intelligence and understanding the right to withdraw from compulsory religious education and collective worship are incompatible with the child's human rights," it said.
It went on to recommend that children who have shown sufficient maturity, intelligence and understanding should be able to choose whether or not to attend assembly.
Terry Sanderson, president of the NSS, said he was often approached by parents whose children were forced to attend collective worship, regardless of their personal beliefs.
"We tend to think it's outrageous that people in Saudi Arabia are forced into Islamic worship," he said. "But we are quite happy to force children here to take part in worship at school. This is about force and coercion. It's an obvious abuse of human rights."
Until 2006, all pupils were obliged to attend collective worship. Since then, over-16s have had the right to exempt themselves.
The Department for Children, Schools and Families announced in February that it would be carrying out a review that would separate guidance on teaching RE from rules about collective worship.
It said it did not intend to change the rules on worship, but did not rule it out in the future.
Responding to the committee's suggestion that pupils should be allowed to withdraw from RE lessons, Ed Balls, Children, Schools and Families Secretary, said: "We do not believe that it is practicable to require schools to conduct the individual assessments that a right to withdraw . would require."