The highest court in Australia has overturned a law allowing same-sex marriage in the Australian Capital Territory.
The federal government challenged the measure which had been passed by the regional parliament in October this year, the BBC reports.
The government said it contravened the federal Marriage Act, which defines marriage as between a man and a woman.
Australia's first same-sex weddings were held in Canberra last weekend when 27 couples tied the knot. Those marriages will now be annulled.
Federal Attorney-General George Brandis had previously warned that the local law would face a legal challenge because it was inconsistent with the Marriage Act.
Prime Minister Tony Abbott is opposed to same-sex marriage.