Millions of women across the US could soon lose their legal right to abortion, according to a leaked Supreme Court document.

The document, published by Politico, suggests the country's top court is poised to overturn the 1973 decision that legalised abortion nationwide.

If the court strikes down the Roe v Wade ruling, individual states would be allowed to ban abortion if they wish.

It is expected abortion could then be banned in almost half of US states.

The Supreme Court's justices are expected to issue a ruling in late June or early July.

Roe v Wade is in the court's sights because Mississippi is asking for it to be overturned. The justices heard that case in December.

Thirteen states have already passed so-called trigger laws that will automatically ban abortion if Roe is overruled this summer. A number of others would be likely to pass laws quickly.

Some 36 million women could then lose abortion access, according to research from Planned Parenthood, a healthcare organisation which provides abortions.

"If the court does overturn Roe, it will fall on our nation's elected officials at all levels of government to protect a woman's right to choose," President Joe Biden said in a statement on Tuesday.

"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," he said.

Anti-abortion groups such as the Susan B. Anthony List have welcomed the news. "If Roe is indeed overturned, our job will be to build consensus for the strongest protections possible for unborn children," it said.

The leaked document - labelled "1st Draft" - appears to reflect the majority opinion of the court, and Politico reports that it was written by Justice Samuel Alito and circulated within the court on 10 February.

But it is unclear if it represents a final opinion, as justices have previously changed their views during the drafting process. The Supreme Court and the White House have not yet commented.

Its publication on Monday night sparked an immediate outcry from Democrats, as well as protests by both pro-choice and anti-abortion campaigners outside the court in Washington DC.

Leading Democratic politicians Nancy Pelosi and Chuck Schumer said jointly that if the report was accurate, the Supreme Court was "poised to inflict the greatest restriction of rights in the past 50 years".

Democratic governors of several states including California, New Mexico and Michigan announced plans to enshrine abortion rights within their constitutions even if the court overturned Roe v Wade.

"I promise you this: I will fight like hell to make sure abortion remains safe, legal and accessible in our state," Michigan Governor Gretchen Whitmer tweeted.

News outlet Politico published the leaked document in full, quoting Justice Alito as saying: "Roe was egregiously wrong from the start. Its reasoning was exceptionally weak, and the decision has had damaging consequences.

"And far from bringing about a national settlement of the abortion issue, [it has] enflamed debate and deepened division."

Rival groups of protesters continued demonstrations outside the Supreme Court on Tuesday, with anti-abortion activists chanting "Roe v Wade has got to go" and abortion rights supporters shouting "abortion is healthcare".

"They are not going to get away with this," pro-choice campaigner Reverend Wendy Hamilton told Reuters news agency. "There is more of us than there is of them, and we are going to fight."

"This is just tearing apart everything that we've worked for," another demonstrator said.

One anti-abortion supporter, Emma Heussner, said: "I'm very passionate about being pro-life. It's a big deal to see Roe v Wade potentially overturned."

The Supreme Court has been reshaped by three appointments under former President Donald Trump, and has been called the most conservative-leaning in modern US history.

Six of the nine current justices were appointed by Republican presidents. The other three were picked by Democratic presidents. The court needs a majority to set a ruling.

Politico reports that Justice Alito and four other Republican-appointed justices - Clarence Thomas, Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett - are in favour of the move against Roe v Wade but it is not clear how Chief Justice John Roberts will vote.

According to the BBC's US partner CBS News, the leak itself will also cause tremendous damage to one of America's most respected institutions and likely see a full-blown investigation, involving the FBI, to unmask the source.

Roe v Wade in 1973 gave women in the US an absolute right to an abortion in the first three months of pregnancy, and limited rights in the second trimester.

But in the decades since, anti-abortion rulings have gradually pared back access in more than a dozen states.

In 2021 alone, nearly 600 abortion restrictions were introduced nationwide, with 90 enacted into law. That is more than in any year since Roe.

Limiting abortion access will most intensely affect poor women, researchers say, and they are already more likely to seek an abortion in the first place.

Women in their 20s account for the majority of abortions - in 2019 about 57% were in this age group. Black Americans get abortions at the highest rate - 27 per 1,000 women aged 15-44.