Former Australian Prime Minsiter Tony Abbott has claimed that same-sex marriage would cause the erosion of family – in a speech to an anti-gay group.

Mr Abbott, a strong opponent of LGBT rights, served as PM until September when he was ousted by his own party.

He headed to the US this week to address the extremist lobbying group Alliance Defending Freedom (ADF) – despite the group allegedly pressuring countries around the world to keep sodomy laws banning gay sex.

The PM pushed ahead with his plans to speak to the group, which has also opposed LGBT people in the military, same-sex adoption and equal marriage, and delivered a fiery speech despite criticism in Australia.

He claimed: “We shouldn’t try to change something without understanding it, without grasping why it is that one man and one woman open to children until just a very few years ago has always been considered the essence of marriage and the heart of family.

“We can’t shirk our responsibilities to the future, but let’s also respect and appreciate values and institutions that have stood the test of time and pass them on, undamaged, when that’s best. That’s a goal we should all be able to share.”

“Policymakers shouldn’t be judgmental about people’s personal choices… but we can’t be indifferent to the erosion of family given its consequences for the wider community.

Citing a predecessor, he claimed “the traditional family was the best social welfare system that mankind has ever devised.”

However, Mr Abbott signalled that despite his own opposition, he would cut his losses and embrace same-sex marriage if his country votes to approve it in a planned plebiscite.

He said: “This is the best way to decide something that’s so important but so personal.

“It’s to let the people decide so that the decision, whichever way it goes, will have their authority.”

His speech to the anti-gay group does not seem to have gone down well with his sister Christine Forster, who is engaged to her same-sex partner

After her brother’s speech, she tweeted: “Marriage is good for our society. That’s why it’s better for all of us if more can be married.

“Allowing same sex couples to marry doesn’t damage the institution in any way. It honours it.”

Ms Forster regularly called out her brother’s approach on same-sex marriage while he was Prime Minister – understandable, given his militant opposition at the time prevented her own wedding from going ahead.