Christians who think they should be exempt from equality legislation are like Muslims trying to impose sharia law on Britain, the chairman of the Equality and Human Rights Commission has said.
Trevor Phillips said faith groups that provide a public service must abide by the law.
Speaking at a debate in London on diverse societies last Wednesday, Mr Phillips singled out Christian adoption agencies that fought legal battles against rules requiring them to treat gay and straight people equally.
"You can't say because we decide we're different then we need a different set of laws," he said.
"To me there's nothing different in principle with a Catholic adoption agency, or indeed Methodist adoption agency, saying the rules in our community are different and therefore the law shouldn't apply to us.
"Why not then say sharia can be applied to different parts of the country? It doesn't work.
"Once you start to provide public services that have to be run under public rules, for example child protection, then it has to go with public law.
"Institutions have to make a decision whether they want to do that or they don't want to do that."
Last year, following a High Court case, the Charity Commission refused to grant an exemption from equality laws for Catholic Care.
The adoption agency, based in the Diocese of Leeds, argued it would have to give up its work finding homes for children if it has to comply with equality regulations which prohibit discrimination against same-sex couples wanting to adopt.
Lord Carey, a former archbishop of Canterbury, described the comparison with sharia as "ridiculous" and called for the Government to find ways of "accommodation" when new laws clash with religion.
He told the Daily Telegraph: "I have argued in the past that there can be only one law to which all should be accountable. But we are not starting with a blank sheet of paper as far as religion is concerned.
"We are a democracy in which Christianity is established in the Church of England and a nation profoundly influenced by this faith in its Catholic and Anglican heritage. We need lawmakers to respect this heritage and seek accommodation wherever a strongly held faith seems to clash with new legislation."