The US Supreme Court ruled on Tuesday that public funds can be used to support families sending their children to religious schools, in a case that challenged longstanding principles of separation of church and state.

The case was brought by two evangelical Christian families in the northeastern state of Maine who sued to be able to use state-provided education subsidy funds to send their kids to schools with religion as the basis of their teachings.

In a 6-3 ruling, the court said religious schools cannot be excluded from the tuition assistance program.

"There is nothing neutral about Maine's program," Chief Justice John Roberts wrote in an opinion joined by the other five conservative justices.

"The State pays tuition for certain students at private schools -- so long as the schools are not religious. That is discrimination against religion," Roberts said.

As Maine is sparsely populated, more than half of its school districts have no publicly funded high schools.

So families receive subsidies that allow them to send their children to schools of their choice, including privately-run schools.

But schools where religious beliefs are at the core of instruction are not covered in the aid program, because, under the regulations for the program -- and those similar in other states -- the teaching is "sectarian."

One of the families wanted to send their children to a school that was disqualified because, local authorities argued, it "teaches children that the husband is the leader of the household" and encourages kids to recognize "God as Creator of the world."

The school chosen by the second family in the case makes use of the Bible in all academic subjects.

And both schools do not accept LGBTQ students or employees.

The parents who sued -- with the backing of Republican senators, 20-odd conservative-led states and many religious institutions -- argued that the Constitution's First Amendment freedom of religion clause gives them the right to choose a school that reflects their values.

If they are denied the subsidies, they claimed, they are suffering discrimination because of their religious views.

The state of Maine argued that under the Constitution government funds cannot be used to support religious schools that discriminate, for example, against LGBTQ students.

'Social conflict'

The administration of President Joe Biden, Democratic-run states and teachers and human rights associations backed Maine.

The three liberal justices on the Supreme Court dissented with Justice Stephen Breyer saying there would be an "increased risk of religiously based social conflict when government promotes religion in its public school system."

Sonia Sotomayor, another liberal, said "what a difference five years makes.

"In 2017, I feared that the Court was 'leading us ... to a place where separation of church and state is a constitutional slogan, not a constitutional commitment," she said.

"Today, the Court leads us to a place where separation of church and state becomes a constitutional violation."

The case is a part of a much wider US debate on the role of parents and school choice in education, and how tax funds dedicated to schools are allocated.

Issues of Covid-19 policies, acceptance of transgender students, and teaching about African-American history have deepened divides and encouraged some parents to look to private and religious schools for their children.