Australia's conservative national government has overruled a local law allowing same sex marriage laws.
Attorney General Philip Ruddock said federal law made it clear that marriage was only between men and women.
The Australian Capital Territory (ACT) which encompasses the country's capital - Canberra - voted last month to allow same-sex unions.
Mr Ruddock ruled that the law would become invalid from midnight tonight.
"From then, there will be no legal basis for the formation of civil unions in the Australian Capital Territory," Ruddock said in a statement.
The government's move to overrule the ACT laws - which would have given gay couples the same legal rights as married couples by describing the marriages as "civil unions" - mirrors the push by US President George W Bush to outlaw gay marriage.
Bush wants the US Senate to pass a constitutional ban on same-sex marriage after several state courts since 2003 ruled that state legislatures did not have the right to ban it.
Canada became the fourth country to legalise same-sex marriage in mid-2005, but its new conservative government will allow a free vote in parliament to decide whether the laws should be overturned.
Australia's centre-left Labor opposition has said the government was wrong to intervene over laws passed by the democratically elected ACT government, which administers the nation's capital Canberra.