Australia's conservative national government has overruled a local law allowing gay unions.
Mr Howard's government formalised marriage laws in 2004
The Australian Capital Territory (ACT) became the first part of the country to legally recognise gay relationships when it voted on the issue last month.
But now the federal government has stepped in to invalidate the new law.
Attorney General Philip Ruddock said that federal law clearly defined marriage as being only between a man and a woman.
He added that the legislation would be invalid from midnight (1400 GMT) on Tuesday.
The ACT law ruled that same-sex couples could be given the same rights as heterosexual couples, in an established relationship separate to marriage.
But many members of Prime Minister John Howard's conservative government opposed the change.
The government formalised federal marriage laws in 2004 to ensure that only men and women could marry.