Australia's main opposition party on Monday proposed to legalize same-sex marriage, making Labor leader Bill Shorten the first head of a major political force in the country to back a bill to overturn a national ban on such unions, The Associated Press reported.

The effort is unlikely to garner much support among conservatives, whose leader, Prime Minister Tony Abbott, is a former Roman Catholic seminarian and staunch opponent of marriage equality, the newswire added.

But Shorten informed lawmakers that he would introduce his bill on Monday, The Australian noted.

"I believe the time has well and truly come for the Parliament to debate marriage equality," the Labor leader noted. "Our current law excludes some individuals - and to me, that is unacceptable (because) it says to them, 'Your relationships are not equally valued by the state, your love is less equal under the law.'"

Under the proposed law, religious clerics would not be required to officiate at same-sex marriages to which they objected, Shorten told his colleagues. He urged Abbott to allow members of the majority Liberal Party to vote their conscience and not require them to back the party's line in opposing marriage equality, the AP detailed.

Abbott has said that the issue is not a priority for his government, which he insisted was focused on national security and the economy. A timeline for a vote on Shorten's proposal, meanwhile, has not been set, though it is likely to come before the end of the year.

Opinion polls show that most Australians support same-sex marriage, and LGBT advocates note that their nation is now the only English-speaking developed country to ban such unions. "Inside the Abbott family, I'm probably the last holdout for the traditional position," the prime minister admitted, according to The Australian.

Members of his party have suggested the country might follow Ireland's example and hold a referendum on the issue, a move Abbott opposes. 

"Referendums are held in this country where there's a proposal to change the constitution; I don't think anyone is suggesting the constitution needs to be changed in this respect," he said.