The U.S. Supreme Court upheld the nationwide ban on a controversial abortion procedure Wednesday, handing abortion opponents the long-awaited victory they expected from a more conservative bench.

The 5-4 ruling said the Partial Birth Abortion Ban Act that Congress passed and President George W. Bush signed into law in 2003 does not violate a woman's constitutional right to an abortion.

The opponents of the act "have not demonstrated that the act would be unconstitutional in a large fraction of relevant cases," Justice Anthony Kennedy wrote in the majority opinion.

The administration defended the law as drawing a sharp line between abortion and infanticide.

The decision pitted the court's conservatives against its liberals, with Bush's two appointees, Chief Justice John Roberts and Justice Samuel Alito, siding with the majority.

Justices Clarence Thomas and Antonin Scalia also were in the majority.

It was the first time the court banned a specific procedure in a case over how — not whether — to perform an abortion.

Abortion rights groups as well as the leading association of obstetricians and gynecologists have said the procedure sometimes is the safest for a woman.

They also said that such a ruling could threaten most abortions after 12 weeks of pregnancy, although government lawyers and others who favour the ban said there are alternate, more widely used procedures that remain legal.

The outcome is likely to spur efforts at the state level to place more restrictions on abortions.

"I applaud the court for its ruling today, and my hope is that it sets the stage for further progress in the fight to ensure our nation's laws respect the sanctity of unborn human life," said Rep. John Boehner of Ohio, Republican leader in the House of Representatives.

But Eve Gartner of the Planned Parenthood Federation of America said the ruling "flies in the face of 30 years of Supreme Court precedent and the best interest of women's health and safety."

"This ruling tells women that politicians, not doctors, will make their health-care decisions for them," she said. 

Most abortions not affected

More than one million abortions are performed in the United States each year, according to recent statistics. Nearly 90 per cent of those occur in the first 12 weeks of pregnancy and are not affected by Wednesday's ruling.

Six federal courts have said the law that was in focus Wednesday is an impermissible restriction on a woman's constitutional right to an abortion.

The law bans a method of ending a pregnancy, rather than limiting when an abortion can be performed.

"Today's decision is alarming," Justice Ruth Bader Ginsburg wrote in dissent. She said the ruling "refuses to take … seriously" previous Supreme Court decisions on abortion.

The procedure at issue involves partially removing the fetus intact from a woman's uterus, then crushing or cutting its skull to complete the abortion.

The law allows the procedure to be performed when a woman's life is in jeopardy.

Abortion opponents say the law will not reduce the number of abortions performed because an alternate method — dismembering the fetus in the uterus — is available and, indeed, much more common.