THOUSANDS of schoolchildren are being forced to sing an alternate version of the Australian national anthem that installs "Christ" as the country's head of state and removes any reference to the Southern Cross.

In a move that outraged parents' groups have labelled "disrespectful", some 50 Christian schools of mixed denominations have replaced the second verse of Advance Australia Fair with the lyrics, which begin, "With Christ our head and cornerstone, we'll build our nation's might", for school assembly renditions.

The contentious version was penned 23 years ago by Sri Lankan immigrant Ruth Ponniah, 75, who now lives in Sydney, as part of her local church's bicentennial celebrations and is now sung in schools including the Penrith Christian School, St George Christian School, Westmead Christian Grammar School and Bethel Christian School in Mt Druitt.

Federal Education Minister Peter Garrett has admonished the unauthorised words, saying that under national protocols the anthem should not be modified and that the alternative verse had no place in the state's educational institutions - regardless of their religious affiliations.

"Alternative words should not be used and schools should be teaching students to sing the two authorised verses," he said.

But the Australian Parents Council, representing the parents of independent school students, wants Mr Garrett to go further and ban Ms Ponniah's lyrics.

"If you're singing the national anthem, it should be the national anthem you are singing," said executive director Ian Dalton.

"There are many opportunities to express pride in your faith, but the national anthem is not one of them. It shouldn't be tampered with."

Federation of Parents and Citizens' Association of NSW president Helen Walton backed the APC push, saying it was "disrespectful" of schools to endorse a different version of the anthem.

"The national anthem is the national anthem of all Australians regardless of religion," she said.

But Christian Schools Australia chief executive officer Stephen O'Doherty said: "Our schools sing it with gusto and I . . . find it inspiring to hear young voices singing those words. We encourage schools to sing this Christian response and to sing it loudly."

Ms Ponniah is also proud it is being sung in schools.