A federal judge has ruled that Mississippi clerks cannot use religious beliefs as a reason not to issue marriage licenses to gay couples, despite a bill passed by the state legislature intended to carve out that exception for them.

The ruling on Monday stops the state from enacting part of a religious objections bill that was due to become law on Friday.

US District Judge Carlton Reeves said that the recusals on religious grounds granted by the state's so-called "Protecting Freedom of Conscience from Government Discrimination Act" violated the US Supreme Court's landmark 2015 ruling legalizing gay marriage.

Reeves said clerks are required to provide equal treatment to all couples, both gay and straight.

The Supreme Court's decision is commonly referred to as the "Obergefell" case after lead plaintiff James Obergefell.

"Mississippi's elected officials may disagree with Obergefell, of course, and may express that disagreement as they see fit - by advocating for a constitutional amendment to overturn the decision, for example," Reeves wrote in his 16-page ruling, which came in response to a lawsuit filed by the Campaign for Southern Equality.

"But the marriage license issue will not be adjudicated anew after each legislative session," Reeves wrote.

Mississippi is among a handful of Southern US states on the front lines of legal battles over equality, privacy and religious freedom.

Reeves has not yet ruled on other provisions of the state legislation, which is expected to become law on Friday and also contains a set of religious objections provisions that have been challenged in four separate lawsuits.

But Mississippi's lieutenant governor, Tate Reeves, quickly slammed the ruling in a written statement.

"If this opinion by the federal court denies even one Mississippian of their fundamental right to practice their religion, then all Mississippians are denied their 1st Amendment rights," Reeves said. "I hope the state's attorneys will quickly appeal this decision to the 5th Circuit to protect the deeply held religious beliefs of all Mississippians."

A spokesman for the Campaign for Southern Equality, meanwhile, said the group was "delighted" with the decision and expected the judge to rule in their favor on its challenges to the entirety of the "Protecting Freedom of Conscience from Government Discrimination Act".

Additional reporting by Reuters.