It is the first time the court has intervened in the way abortions are carried out, as opposed to just over abortion itself.

The decision was viewed by both pro and anti-abortionists as an indication of a changing mood in the court towards a more conservative position.

"We're moving beyond putting roadblocks in front of abortions to actually prohibiting them," said Troy Newman, president of Operation Rescue, a national anti-abortion group.

He and other strategists said they hoped to introduce bills in a number of states that would:

- Ban all abortion of viable foetuses, unless the mother's life is endangered.
- Ban mid and late-term abortion for foetal abnormality, such as Down syndrome or a malformed brain.
- Require doctors to tell patients explicitly what an abortion will involve and show them ultrasound images of the foetus.
- Lengthen abortion waiting periods so women must reflect on such counselling.

It is far from certain that the Supreme Court would uphold all these proposals, but anti-abortion activists feel momentum is on their side.

For more than 30 years, since the Roe v Wade case in 1973 established abortion as a constitutional right, the court has tended to reject any attempt to restrict access to terminations.

What appears to have been crucial in tipping the court's attitude was the retirement last year of Sandra Day O'Connor from the nine-member panel, and her replacement, at President George Bush's instigation, by the more conservative Samuel Alito.

Abortion-rights lawyer Katherine Grainger predicted the ruling would "open the floodgates" in state after state.

"The state's interest in the foetus has now been elevated above the woman's health," said Ms Grainger, who directs state policy for the Centre for Reproductive Rights. "States are going to push the boundaries and try to restrict access on all fronts."

The procedure that was challenged involves doctors partially removing the foetus from a woman's uterus and then crushing or cutting into its skull while it is still in the woman's body. Partial-birth abortions are allowed only for medical reasons and are applied to terminations after 12 weeks.