WASHINGTON (REUTERS) - The US Supreme Court late on Wednesday (Sept 1) refused to block a Texas ban on abortion after six weeks of pregnancy, dealing a major blow to abortion rights and allowing a law prohibiting the vast majority of abortions in the state to remain in place.

The justices on a 5-4 vote denied an emergency request by abortion and women's healthcare providers for an injunction barring enforcement of the ban, which went into effect early on Wednesday, while litigation continues in their lawsuit challenging its constitutionality.

The decision saw one of the court's six conservatives, Chief Justice John Roberts, join the three liberals in dissent.

"The court's order is stunning," wrote liberal Justice Sonia Sotomayor in a dissenting opinion.

"Presented with an application to enjoin a flagrantly unconstitutional law engineered to prohibit women from exercising their constitutional rights and evade judicial scrutiny, a majority of justices have opted to bury their heads in the sand," she added.

The court's majority, in an unsigned explanation, said the decision "is not based on any conclusion about the constitutionality of Texas's law" and allows legal challenges to move forward.

The court's action represents a major milestone in the fight over abortion as opponents have sought for decades to roll back access to the procedure and its legality.

The law would amount to a near-total ban on the procedure in Texas, as 85 per cent to 90 per cent of abortions are obtained after six weeks of pregnancy, and would likely force many clinics to close, abortion rights groups said.

Such a ban has never been permitted in any state since the Supreme Court decided Roe v Wade, the landmark ruling that legalised abortion nationwide, in 1973, they said.