The US Supreme Court could be about to overturn the nationwide legal right to abortion, according to an unprecedented leaked draft of a court document.

In a 98-page draft opinion, Justice Samuel Alito writes that the 1973 Roe v Wade decision legalising abortion across the US is "egregiously wrong".

If the top US court strikes down the ruling, "trigger laws" could instantly make abortion illegal in 22 US states.

The justices are not expected to issue a ruling until early July.

The 1973 law is in the court's sights because it is weighing up a challenge to Mississippi's ban on abortion after 15 weeks, which the justices heard in December.

But the reported threat to Roe v Wade comes at a time when reproductive rights are being threatened in Republican leaning states around the US.

And it sparked immediate outcry from Democrats, and protests - by both pro and anti-abortion supporters - outside the Supreme Court on Monday night.

Speaker of the House of Representatives Nancy Pelosi and Senate Majority Leader Chuck Schumer - both Democrats - issued a joint statement saying that if the report was accurate, the "Supreme Court is poised to inflict the greatest restriction of rights in the past 50 years".

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, Roe and Casey have enflamed debate and deepened division."

Neither the Supreme Court nor the White House have commented on the leak.