A federal judge in San Francisco struck down the Partial Birth Abortion Ban Act yesterday, ruling that the law jeopardizes other legal forms of abortion and threatens the health of women who end their pregnancies.

In a strongly worded opinion that accused Congress of misrepresenting many scientific facts about the procedure, U.S. District Judge Phyllis J. Hamilton concluded that the bill -- approved by wide margins in Congress last year and signed by President Bush -- was unconstitutional.

Hamilton adopted most of the arguments put forward by the Planned Parenthood Federation of America in finding that the measure was too vague, that it placed an unfair burden on women seeking certain kinds of abortions and that it did not do enough to protect women's health.

The ban on the procedure that critics call "partial birth abortion" was already on hold temporarily as three courts heard legal challenges to it, but Hamilton's decision specifically prohibits the Justice Department from enforcing the law at any of Planned Parenthood's 900 clinics, which perform about half the nation's abortions. Planned Parenthood physicians who perform the procedure outside the organization's clinics also are protected. Two other challenges to the law are expected to be decided by other federal judges this summer.

Doctors call the procedure "intact dilation and extraction." Developed in the early 1990s, it requires partly delivering an intact fetus, typically during the second trimester, and piercing and compressing skull. It is unclear how many abortions are performed this way each year, but it is a small percentage.

Many abortion providers argue that it is necessary because it can be lifesaving for some women and can protect the fertility of others. Opponents say that it ends the life of a being that had at least partly left its mother's body.

The ruling was welcomed as a major victory by Planned Parenthood President Gloria Feldt, who said it "reaffirmed a woman's right to choose, and a doctor's right to practice medicine." She also said the ruling was appropriately critical of Congress. "Because there's a herd mentality on a bill doesn't make it right. The legislation is fatally flawed constitutionally."

Monica Goodling, spokeswoman for the Justice Department, which is defending the law in all three cases, said yesterday that the department is reviewing the ruling.

"The Justice Department vigorously litigated this case, as well as the pending cases in Nebraska and New York involving challenges to the law banning partial-birth abortions," she said in a statement. "The Department will continue to devote all resources necessary to defend this Act of Congress, which President Bush has said 'will end an abhorrent practice and continue to build a culture of life in America.' "

In 2000, the U.S. Supreme Court voted 5 to 4 to overturn a Nebraska law that outlawed the procedure, ruling that it was unconstitutional because it did not allow the abortion method even if a doctor believed it was the safest way to preserve a woman's health. The federal law passed in 2003 also provides no exemption if a woman's health is at stake. Congress sidestepped the Supreme Court's earlier ruling by issuing "findings of fact," including the conclusion that these late-term abortions are never medically necessary.

Hamilton disputed that decision, citing U.S. Supreme Court Justice Clarence Thomas's opinion in a separate case that courts can overrule legislative judgments on what is fact and what is not.

She wrote, for instance, that by referring to the procedure as "infanticide," Congress was being "grossly misleading and inaccurate." She said that Congress was aware that the abortion procedure banned by the bill applied to fetuses that were too young to live outside the womb.

Sen. Dianne Feinstein (D-Calif.), who opposes the law, said: "The vague wording of the bill -- the use of the term 'partial birth' as opposed to writing in a specific medical procedure -- was really a stalking horse for anti-choice advocates to impact second-trimester abortions."

Proponents of the law, which was approved by the Senate 64 to 33 and by the House 282 to 139, called the decision unfortunate but expected. Sen. Rick Santorum (R-Pa.), a leading opponent of abortion rights, called the ruling "a travesty," saying Hamilton was deeply biased against the law.

"She is a very liberal judge," he said. "This is a classic example of how judges impose their philosophies on judicial proceedings. . . . She clearly had prejudged this case."

Douglas Johnson, legislative director of the National Right to Life Committee, also criticized the judge, who was appointed by President Bill Clinton. "Judge Hamilton's deep personal hostility to the law has been evident throughout the judicial proceedings, and is evident in many passages in her 117-page injunction," he said. "It is the U.S. Supreme Court that will ultimately decide whether our elected representatives can ban the practice of mostly delivering a living premature infant and then puncturing her skull."

The California case was brought by Planned Parenthood. The New York case was filed by the National Abortion Federation, which represents nearly half the nation's abortion providers. The Nebraska case was brought by several doctors who perform abortions.

The Nebraska case is before U.S. District Judge Richard Kopf, who ruled in 1997 that the state's ban on intact dilation and extraction was unconstitutional. In New York, the case is before U.S. District Judge Richard Conway Casey. However those judges rule, the case is expected to be appealed and most likely decided by the Supreme Court.

Staff writers Charles Babington and Kimberly Edds contributed to this report.