US political website Politico says it has obtained a draft Supreme Court majority opinion that would overturn a decision that led to the legalization of abortion nationwide.

The top federal court is set to rule on the constitutionality of a Mississippi law that bans most abortions after 15 weeks of pregnancy. A decision is expected as early as this summer.

The Supreme Court made the landmark decision in 1973 protecting women's right to choose abortion.

Politico on Monday published what it called an initial draft majority opinion, which it says is a "full-throated, unflinching repudiation of the 1973 decision," allowing each state to decide whether to restrict or ban abortion.

The site says "Justices can and sometimes do change their votes as draft opinions circulate." It also said "The court's holding will not be final until it is published."

It is thought many of the nine justices of the Supreme Court have a negative view on abortion rights, as six of them are conservatives.

After major US media reported the article, abortion rights supporters and pro-life activists gathered in front of the top court in Washington, even though it was late at night.

Media reports said an authentic leak of such a document involving the Supreme Court would be unprecedented.