Plans for new faith schools in England to admit up to 25% of pupils from other religions "must be resisted", the Archbishop of Birmingham has said.
Archbishop Nichols has written to 2,075 heads of Catholic schools
The most Rev Vincent Nichols described the plans as "insulting" and "divisive" and has urged the head teachers of Catholic schools to voice their fears.
The plans were introduced in an amendment to the Education and Inspections Bill last week.
The government has said schools are in a position to prevent social division.
Education Secretary Alan Johnson met with representatives from the UK's major religious groups on Monday for a so-called "inclusion summit" to discuss the role faith schools can play in improving relations between the faiths.
The Department for Education and Skills said the meeting had been productive and Mr Johnson had made it clear that the amendment would only apply to new faith schools.
He also explained that where there is local opposition, a local authority will need the consent of the education secretary to approve a new faith school with fewer than 25% of non-faith admissions.
The Church of England has said its new schools will admit up to 25% of pupils from outside the faith - but said other religions should not be expected to offer the same commitment.
But the amendment has met with opposition from Muslim, Jewish and Catholic groups.
Writing in the Telegraph newspaper, the archbishop said coercive measures by the government would not win co-operation and branded them "ill-thought out, unworkable and contradictory of empirical evidence".
He said Catholic schools on average welcome 30% of pupils from other faiths or none, and they were likely to have better academic records and less likely to encounter bullying or racism.
He added that the government appears to hold the view that, left to themselves, Catholic schools would be divisive.
"Since the evidence suggests the opposite, I can only assume that this view rises from muddled thinking or prejudice," he wrote.
He warned: "The introduction of 'admissions requirements' is a Trojan horse, bringing into Catholic schools those who may not only reject its central vision but soon seek to oppose it."
The way forward, he said, was a "mutually respectful co-operation" between faith groups and authorities.
But this amendment, he warned "seems to signal an alternative and deeply divisive step. It has to be resisted."
Last week, he wrote to the head teachers of 2,075 secondary and primary Roman Catholic schools urging them to write to their MPs to voice their concerns.
ENGLAND'S FAITH SCHOOLS
Church of England 4,646
Roman Catholic 2,075
He has also called for talks between the government and the Catholic church.
Rabbi James Kennard, head teacher at King Solomon High School in Ilford, Essex, shared his view, saying Jewish schools had not been able to explain their position.
In an interview with the Guardian newspaper, he said: "The Jewish school is the traditional institution where a youngster's Jewish identity is shaped, through an all-embracing ethos that runs alongside, and integrates with, the educational requirements of the country where Jews are living.
"The Jewish community is small, needs to maintain its distinct identity and ethos and has no interest in spreading its message to others."
He added that when people have a good grounding in their religion, they tend to be able to participate in wider society.
The Department for Education said it welcomed the steps faith groups have already taken to improve community cohesion and said they were talking to them about how to build on this.