The Roman Catholic Church squared up for a political battle with the Government yesterday as it warned the Cabinet that it could not accept a law forcing it to embrace adoption by gay couples.

Cardinal Cormac Murphy-O’Connor, the Archbishop of Westminster, took the unusual step of writing to every Cabinet member saying that he would have “serious difficulty” with such a move.

Catholic teaching about family life meant that its adoption agencies “would not be able to recruit and consider homosexual couples as potential adoptive parents”, he added. “We believe it would be unreasonable, unnecessary and unjust discrimination against Catholics for the Government to insist that, if they wish to continue to work with local authorities, Catholic adoption agencies must act against the teaching of the Church and their own consciences,” the Archbishop told them.

Such a strongly worded intervention by the head of the Catholic Church in England and Wales will further stoke a row within the Government over new discrimination regulations.

The Times has been told that Tony Blair sided with the Catholic Church and expressed fears that its adoption agencies would close unless he allowed an exemption when Angela Eagle, a Labour MP, tackled him over the issue at the parliamentary committee of Labour backbenchers last week.

The Sexual Orientation Regulations have aroused opposition among some evangelical Christian leaders because of fears that Christian hotel owners could be open to prosecution if they refused to let homosexual couples have a room.

But a spokesman said that the new regulations would not affect the existing policy of Anglican adoption charities, where couples and individuals are assessed according to their suitability and to the needs of the child.

In its response to the government consultation, the Church said that it was crucial to ensure that churches and other faith communities were able to manifest their convictions and doctrines without fear of prosecution. “That means that the regulations need to strike a careful balance, as in other anti-discrimination legislation, between potentially competing rights,” it added.

Ruth Kelly, the Communities Secretary, who is responsible for the regulations as the Minister for Equality Policy, is said by colleagues to have been ready to propose an exemption that would allow Catholic agencies to continue as they are.

But her stance has provoked a revolt by other ministers and Labour backbenchers, who say that an exemption would allow discrimination against homosexuals.

Chris Bryant, Labour MP for Rhondda, said: “I suspect the bishops are hopelessly out of step with ordinary Catholics on this. Surely all that counts is the best interests of the child.”

The row has echoes of the last big intervention into politics by the Roman Catholic Church, when it began a lobbying campaign that forced the Government to abandon plans last autumn to require new faith schools to accept a quarter of their admissions from children of other religions.

This time the church is threatening to close 12 Catholic adoption and fostering agencies unless they win an exemption.

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