The US Supreme Court has confirmed that a leaked draft ruling which could strike down the right to abortion is authentic as President Joe Biden vowed to protect women’s rights.

The draft copy of a majority opinion written by Justice Samuel Alito would overturn the landmark 1973 Roe v Wade decision which legalised abortion.

In a news release the court said it would launch an investigation into how the copy was leaked to the news outlet Politico, adding that the document did not represent “the final position of any member” of the high court.

“I have directed the Marshal of the Court to launch an investigation into the source of the leak,” Chief Justice John Roberts said, calling it an “egregious breach” of trust.

“To the extent this betrayal of the confidences of the Court was intended to undermine the integrity of our operations, it will not succeed. The work of the Court will not be affected in any way,” he said in a statement.

The leak sparked immediate public outcry amid fears that if it was passed, abortion would become illegal in 20 US states because of trigger laws.

The abortion ruling, due by the end of June, would be the court’s biggest since former president Donald Trump succeeded in naming three conservative justices to the court — Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett.

Speaking on the steps of the US Capitol, Democrat Senate Majority Leader Chuck Schumer said it was a “dark and disturbing day for America” and an “abomination, one of the worst in modern history”.

The public reaction to the draft anti-abortion ruling was immediate. Photo: Getty

Democrats at the state and federal level and abortion rights activists were looking for some way to head off the sweeping social change long sought by Republicans and religious conservatives.

Mr Biden said voters would need to elect more members of Congress who support abortion rights so that they could pass legislation making Roe v Wade the law of the land.

The Roe decision recognised that the right to personal privacy under the US constitution protects a woman’s ability to terminate her pregnancy.

“At the federal level, we will need more pro-choice Senators and a pro-choice majority in the House to adopt legislation that codifies Roe, which I will work to pass and sign into law,” Mr Biden said.

Democratic-backed legislation to protect abortion access across the US died in Congress this year as the razor-thin majority held by Mr Biden’s party was insufficient to overcome Senate rules requiring a supermajority to move forward on most legislation.

Democrats tend to support abortion rights while Republicans tend to oppose them.

“Do something, Democrats,” abortion rights protesters chanted as they rallied outside the court in the pre-dawn hours against the decision, which would be a triumph for Republicans who spent decades building the court’s current 6-3 conservative majority.

“If the report is accurate, the Supreme Court is poised to inflict the greatest restriction of rights in the past fifty years, not just on women but on all Americans,” the top two Democrats in Congress — House of Representatives Speaker Nancy Pelosi and Senator Schumer — said in a joint statement.

US Senate Majority Leader Senator Chuck Schumer with Democratic Senate members on the steps of the US Capitol. Photo: Getty

Senate Republican Leader Mitch McConnell condemned the leak as a “lawless action” that should be “investigated and punished as fully as possible”.

Senator McConnell said the Justice Department must pursue criminal charges if applicable.

In the absence of federal action, states have passed a raft of abortion-related laws.

Republican-led states have moved swiftly, with new restrictions passed this year in six states.

Three Democratic-led states this year have passed measures intended to protect abortion rights.

Abortion is one of the most divisive issues in US politics and has been for nearly a half century.

A 2021 Pew Research Center indicated that 59 per cent of US adults believed it should be legal in all or most cases while 39 per cent thought it should be illegal in most or all cases.

The anti-abortion group the Susan B Anthony List welcomed the news.

“If Roe is indeed overturned, our job will be to build consensus for the strongest protections possible for unborn children and women in every legislature,” its president Marjorie Dannenfelser said.

“Roe was egregiously wrong from the start,” conservative Justice Samuel Alito wrote in the draft opinion, which is dated February 10, according to Politico, which posted a copy online.

The case involves a Republican-backed Mississippi ban on abortion starting at 15 weeks of pregnancy, a law blocked by lower courts.

Based on Alito’s opinion, the court would find that the Roe v Wade decision that allowed abortions performed before a foetus would be viable outside the womb — between 24 and 28 weeks of pregnancy — was wrongly decided because the constitution makes no specific mention of abortion rights.

“Abortion presents a profound moral question. The Constitution does not prohibit the citizens of each state from regulating or prohibiting abortion,” Alito said, according to the leaked document.

-with AAP