The Supreme Court is poised to strike down the right to abortion in the United States, according to a leaked draft of a majority opinion that would shred nearly 50 years of constitutional protections.

The draft opinion was written by Justice Samuel Alito and has been circulating inside the conservative-dominated court since February, the news outlet Politico reported.

The leak of a draft opinion, while a case is still pending, is an extraordinary breach of Supreme Court secrecy.

The 98-page draft majority opinion calls the landmark 1973 Roe v. Wade decision enshrining the right to abortion "egregiously wrong from the start."

"We hold that Roe and Casey must be overruled," Alito writes in the document, labelled as the "Opinion of the Court" and published on Politico's website. "It is time to heed the Constitution and return the issue of abortion to the people's elected representatives."

In Roe v. Wade, the nation's highest court held that access to abortion is a woman's constitutional right.

In a 1992 ruling, Planned Parenthood v. Casey, the court guaranteed a woman's right to an abortion until the fetus is viable outside the womb, which is typically around 22 to 24 weeks of gestation.

"Abortion presents a profound moral question," Alito wrote. "The Constitution does not prohibit the citizens of each state from regulating or prohibiting abortion.

"The inescapable conclusion is that a right to an abortion is not deeply rooted in the Nation's history and traditions," he said.

Reproductive rights have been increasingly under threat in the United States in recent months as Republican-led states move to tighten restrictions with some seeking to ban all abortions after six weeks, before many women even know they are pregnant.

Right-wing politicians have launched an assault on abortion, with Democrats, led by President Joe Biden, fighting back to protect access to the procedure.

In December, hearing oral arguments about a Mississippi law that would ban most abortions after 15 weeks, the Supreme Court's conservative majority appeared inclined to not only uphold the law but to toss out Roe v. Wade.

'Crisis moment'

The nine-member court, dominated 6-3 by conservatives following the nomination of three justices by former president Donald Trump, is expected to issue a decision in the Mississippi case by June.

Politico, citing a person familiar with the court's deliberations, said four other conservative justices - Clarence Thomas, Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett - had voted with Alito, the author of the first draft of the majority opinion.

It said the three liberal justices on the court were working on a dissent and it was unknown how Chief Justice John Roberts would ultimately vote.

Politico stressed that the document it obtained is a draft and justices do sometimes change their votes before a final ruling.

The leak of a draft opinion is extraordinary while a case is still being decided. Politico said it was the first time in modern history a draft opinion had been disclosed publicly.

"This is the equivalent of the Pentagon Papers leak, but at the Supreme Court," said Neil Katyal, who served as solicitor general under president Barack Obama, in a reference to the leaked documents outlining US involvement in Vietnam.

"I'm pretty sure there has never ever been such a leak."

Asked about the draft being circulated, a Supreme Court spokeswoman said: "The Court has no comment."

The Guttmacher Institute, a pro-choice research group, has said that 26 states are "certain or likely" to ban abortion if the Roe is overturned.

Liberal states that decide to do so could still legally allow abortion even if the court overturns Roe v. Wade.

Planned Parenthood, which operates abortion clinics around the country, said the draft opinion is "outrageous" but cautioned that it "is not final."

"While abortion is still legal, tonight's report makes clear that our deepest fears are coming true," it added. "We have reached a crisis moment for abortion access."

A number of Democratic lawmakers took to Twitter to express concern about the threat to abortion rights.

"An extremist Supreme Court is poised to overturn Roe v. Wade and impose its far-right, unpopular views on the entire country," said Senator Elizabeth Warren of Massachusetts.

Josh Hawley, a conservative Republican senator from Missouri, welcomed the Politico report.

"If this is the Court's opinion, it's a heck of an opinion," Hawley said. "Voluminously researched, tightly argued, and morally powerful."