Pro-choice activists have rallied against what they see as an assault on abortion rights

Judge Phyllis Hamilton said a law banning late-term abortions was too vague and restricted the right to choose an abortion.

The case was brought by Planned Parenthood, an organisation that runs abortion clinics across the US.

The ruling is a major setback for the Bush administration.

It means the government is not allowed to enforce the law, signed by the president last November.

"This court concludes that the act is unconstitutional because it poses an undue burden on a woman's ability to choose a second trimester abortion; is unconstitutionally vague; and requires a health exception," Judge Hamilton wrote in her ruling.

I'm very disappointed in the decision today, and I think that ultimately it will be overturned Bill Frist Senate Majority leader

As a result, Attorney General John Ashcroft and his employees would be prevented from enforcing the law in Planned Parenthood's clinics across the country, which number more than 900, the ruling added.

Planned Parenthood lawyer Beth Parker welcomed the ruling, saying it sent a "strong message" that the government "should not be intruding on very sensitive and private medical decisions".

'Shameful procedure'

Supporters of the administration reacted with dismay.

"As a physician, this procedure is shameful," said Senate Majority leader Bill Frist, describing it as "outside of the bounds of the practice of medicine".

"I'm very disappointed in the decision today, and I think that ultimately that decision will be overturned."

The law, signed last November, banned so-called "partial-birth abortions", which usually take place during the fifth or sixth month of pregnancy.

The procedure involves the extraction of the body of the foetus into the vagina before the contents of the skull are suctioned and the intact foetus is removed from the woman's body.

Backers of the law say the procedure is a form of infanticide.

Opponents argue that it does not allow exemptions for a woman's health, a caveat cited by former US President Bill Clinton who twice vetoed similar legislation.

There are also concerns that the law marks the beginning of an assault on the Supreme Court's landmark decision Roe v Wade in 1973, which guarantees abortion rights.

The San Francisco decision comes only days after another federal judge blocked attempts by the government to reverse an Oregon state law allowing assisted suicides.