The US Supreme Court confirmed on Tuesday the authenticity of a leaked draft ruling that would overturn four decades of legal abortion protections, but maintained the decision is not final.

The court said the document "does not represent a decision by the Court or the final position of any member on the issues in the case," and Chief Justice John Robert announced he has directed a probe into the exceedingly rare leak.

"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.

"Court employees have an exemplary and important tradition of respecting the confidentiality of the judicial process and upholding the trust of the Court. This was a singular and egregious breach of that trust that is an affront to the Court and the community of public servants who work here," he added.

Roberts directed the court's marshalls to investigate the source of the leak.

The Politico news website published Monday evening a draft majority opinion in which Justice Samuel Alito rules to overturn the landmark 1973 Roe v. Wade case, which cemented abortion rights across the land.

He is joined in the not yet final ruling by Justices Clarence Thomas, Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett, a person familiar with the court's closed-door deliberations told Politico. Democratic appointed Justices Stephen Breyer, Sonia Sotomayor and Elena Kagan are in dissent.

It is not clear which way Roberts will vote, but even without his support the court's conservative wing appears to have sufficient votes to overturn Roe in the nine-member bench.

In the decision, Alito writes that Roe, which has stood for nearly four decades, "was egregiously wrong from the start."

"We hold that Roe and Casey must be overruled," he wrote, referring to the original case and a subsequent 1992 ruling that largely upheld them.

US President Joe Biden maintained in an earlier statement that while it is unclear if the draft will reflect the court's final decision, he believes "a woman’s right to choose is fundamental."

"Roe has been the law of the land for almost fifty years, and basic fairness and the stability of our law demand that it not be overturned," he said in a statement.

"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. And it will fall on voters to elect pro-choice officials this November," he added.

An official decision is expected to be published before the court's current term ends in late June or early July, and the draft could see significant revisions in the interim. Justices could also change their positions as the draft circulates.

Should Roe and Casey be overturned, the matter of abortion rights would fall to individual states to determine. That would almost certainly lead to their complete rollback across wide swathes of the Midwest and South.