The pill has been available in Europe since 1988

"At long last, science trumps anti-abortion politics," she said.

The pill was approved for sale by the US Food and Drug Administration (FDA), and will be marketed and sold under the name Mifeprex by the New York-based Population Council group.

The drug is controversial because it causes an abortion if taken within the first 49 days of pregnancy.

There have also been some concerns over its side-effects, which include heavy bleeding and nausea.

But in a statement, the FDA said the approval was "the result of careful evaluation of the scientific evidence related to the safe and effective use of this drug".

Pro-life groups attacked the decision, and vowed to use fears over the pills' safety in their continuing anti-abortion campaign.

"The new pill will result in more abortions and new risks to women," said a spokeswoman from the National Right to Life commitee.

Highly effective

Mifepristone is available in 13 countries across the world, including Britain, and has been used by millions of women.

Anti-abortion campaigners object to the drug

Studies have shown that the pill, which works by blocking a hormone needed to sustain pregnancy, is 95% effective.

The pill suppresses the body's production of a hormone called progesterone, causing the uterine lining to thin and reject an implanted embryo.

The drug will be available for sale through the National Abortion Federation, which administers terminations in the US.

The federation said 240 of its member clinics were ready to use the drug, which will be available within a month.