A court challenge by the Clinton administration to the all-male Virginia Military Institute threatens the very existence of single-sex colleges in the United States, its opponents say.

And the Supreme Court case, due to be heard in mid-January, is rallying a surprising collection of prominent female lawyers to the defence of a publicly funded military college that has consistently refused to admit women.

Although only about 4 per cent of women graduate from single-sex colleges, they account for a quarter of all women serving in Congress and about one-third of female board members of top US companies. Groups like the Independent Women's Forum, who reject what they regard as knee-jerk feminism, have rallied to VMI's cause.

About 64,000 women attend 83 single-sex colleges, compared to only 11,000 men at 12 mostly little-known institutions, including the 1,300 men at VMI, a small school that prides itself on a gruelling physical regime. No individual woman is trying to enter VMI. Instead, the administration is challenging a lower court's ruling that its men-only policy was defensible because it has set up a "comparable" programme for women.

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The US Justice Department argues that discrimination based solely on sex is "inherently suspect" under the constitution. That would put it on the same footing as racial discrimination - effectively forbidden by law.

The majority of women's rights groups back the administration. They say a victory in this case would cap a long struggle to overturn a historical legacy of sex discrimination that once barred women from attending Harvard and Yale.

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But in practice the remaining single-sex institutions are dominated by women's colleges. Their alumni include Hillary Clinton and her fellow Wellesley College graduate Susan Estrich, a USC law professor who has filed a brief in the case backing VMI.

Recently the liberal orthodoxy that "separate but equal" is simply a euphemism for racism and sexism has undergone some revision as overt racism and sexism in education has ended. Black leaders have worked to keep historically black colleges alive.

VMI is funded by the local state education system. But Donald Alexander, a Republican attorney who served as tax commissioner under Presidents Nixon and Carter, warns that an adverse ruling could threaten tax breaks for private colleges worth billions of dollars.

If VMI's men-only policy is struck down, he said, it was hard to see how single-sex institutions could still qualify for non-profit status, and for rules that allow alumni to give huge tax-deductible gifts.

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The administration's brief holds that "sex, like race, is an immutable and highly visible characteristic that frequently bears no relation to ability to perform or contribute to society".

Professor Estrich, however, recently told the Los Angeles Times: "I don't think there is any doubt that single-sex education has an important role for women. So if you believe that, then you have to ask yourself, 'Why shouldn't boys have the same option?'"