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London — British regulators lack the authority to let couples select test-tube baby embryos to match tissue types of a sick brother or sister, a High Court judge ruled Friday.

The ruling was a setback for a British couple who have attracted widespread publicity in their quest to conceive a fifth child as an umbilical cord blood donor for their seriously ill toddler.

The couple was unavailable for comment Friday. But the British Medical Association said if regulators cannot approve embryo screening for tissue typing to save the life of a sibling, then a law should be passed to permit it.

Josephine Quintavalle, whose organization Comment on Reproductive Ethics brought the case, hailed the High Court's ruling.

"With social sex selection around the corner and innumerable other designer baby possibilities on the horizon, today's judgment is particularly timely," Ms. Quintavalle said. "These vital issues involve the very essence of what it is to be human. Parliament alone is the correct arena for decisions of such magnitude."

In December, 2001, Britain's Human Fertilization and Embryology Authority ruled couples undergoing in vitro fertilization who were having their embryos screened for serious hereditary diseases could also have them tested for tissue matching to save the life of a sibling.

Critics said the decision could lead to babies being created to provide spare parts.

On Friday, Justice Maurice Kay ruled the embryology authority had no legal power to license embryo selection by "tissue typing" to help sick brothers or sisters.

He said the case was in "a difficult area of medical science and ethics" and that the legislation had been "tightly drawn," so the ground rules "restrict the potential for misuse of science and technology."

He noted the House of Commons Select Committee on Science and Technology had criticized the authority's approach to tissue typing.

During in vitro fertilization, several embryos are created but only two or three are implanted in the womb. In some cases, the embryos are tested for certain genetic defects and only those free of abnormal genes are implanted.

Earlier this year, the embryology authority gave Shahana and Raj Hashmi of Leeds permission to add another test to make sure tissue from their next child would be a suitable match for their ailing 2-year-old son, Zain.

Neither the couple nor their four other children are bone marrow matches for Zain, who suffers from the rare blood disorder thalassemia and is expected to die without a transplant. Stem cells taken from the baby's umbilical cord at birth could replace Zain's bone marrow.

The Hashmis have reportedly been trying to conceive a fifth child, without success so far.

Cases of embryo selection for matching tissue for sick siblings have occurred in the United States, where private fertility clinics are unregulated.

A new commission on bioethics was created by U.S. President George W. Bush earlier this year to review and make recommendations on experiments on stem cells from human embryos. The commission is also mandated to look into "assisted reproduction," which primarily means in vitro fertilization.

The Human Fertilization and Embryology Authority is the only regulator of its kind in the world. Its origins are rooted in a passionate debate in Britain over reproductive ethics that erupted after the birth of the world's first test-tube baby in London in 1978.

The furor prompted a landmark report on the ethics and science of reproductive medicine, which eventually led to the establishment in 1991 of the Human Fertilization and Embryology Authority.

The agency keeps tight control over the practices of all fertility clinics and over embryo research.