Australia’s federal government plans to challenge the Australian Capital Territory’s same-sex marriage law in the High court.

It’s another sign of tension on the issue between Prime Minister Tony Abbott’s federal governing Liberal Party, which formally opposes equal marriage, and the ACT’s Labor Government who support the measure.

Labor expects the law to pass with the support of Greens MLA Shane Rattenbury when the ACT Assembly sits later this month.

It means the first same-sex weddings in the territory could take place before Christmas – although equal marriage campaigners fear a legal challenge by the federal government could invalidate same-sex marriages performed after the bill comes into force.

ACT Attorney General Simon Corbell says it is disappointing that the federal government will be challenging the legislation.

Australian Marriage Equality chair and independent New South Wales MP Alex Greenwich said the fact the federal government was intervening would encourage same-sex couples to get married sooner rather than later.

“The more people we have expressing their love and commitment will make it harder for any laws to be overturned,” he told AAP.

“I would encourage as many same-sex couples to get married in the ACT as possible.”

Equal marriage remains banned at a federal level in Australia.

Efforts to legalise the measure failed in the Australian Parliament last year.