Festival of Light Australia, a family advocacy group, has called on the country's Prime Minister John Howard to amend the Marriage Act 1961 in an attempt to stop states from allowing same-sex and other unmarried couples from being registered by the Registrars of Births, Deaths and Marriages.

The Prime Minister John Howard yesterday told a National Summit on Marriage, Family and Fatherhood in Parliament House that children deserve both a mother and a father, and that marriage between a man and a woman is the foundation of our society.

Richard Egan, spokesman for Festival of Light, said at the summit that state-based registration of same-sex and other non-married couples would undermine marriage.

Egan referred to legal opinions obtained by Festival of Light which conclude that the Commonwealth's constitutional power over marriage would support an amendment to the Marriage act 1961 to invalidate any state law attempting to give legal recognition through registration to couple relationships other than marriage.

The Festival of Light spokesman has also this week delivered an open letter with some 6,000 signatures to the Prime Minister, as well as all Labour and Coalition MPs and Senators.

According to the letter, "Research shows that marriage is the most enduring of human sexual relationships - more stable than male-female cohabitation and far more stable than homosexual relationships.

"Children raised by their married parents do better than those raised by cohabiting parents, who do better than children raised by same-sex partners.

"Children brought up in non-marriage contexts have higher rates of physical and mental health problems, child abuse, criminality, drug abuse and poor academic performance.

"Marriage is under attack across the Western world today. Several countries have already given in to pressure from the homosexual lobby to allow same-sex 'marriage' or registration of same-sex relationships. The result has been a decline in respect for marriage with an increase in fragile male-female cohabitation, leading to more children growing up in fatherless homes."

The letter concluded: "Giving same-sex and other non-marital relationships legal recognition equivalent to marriage would undermine the unique status of marriage - with devastating consequences for children and for society."