Lidia Thorpe will quit the Greens and move to the crossbench in a stunning development that will make the firebrand Victorian senator a crucial balance-of-power vote in the upper house.

“I am resigning from the Greens,” Senator Thorpe said in a hastily called media briefing in Canberra on Monday.

“My focus now is to grow and amplify the black sovereign movement in this country.

“I’ll be their voice. I’ve spent my entire life fighting for justice – to save black lives.”

The move comes after the Victorian senator apparently split with other members of the Greens party room over the looming referendum to create an Aboriginal and Torres Strait Islander advisory body to the Parliament.

The Greens appear likely to support the voice. But Senator Thorpe criticises the concept, which she says insults Indigenous sovereignty and needed to be preceded by a treaty with Indigenous Australians.

Only days ago Greens leader Adam Bandt had been at pains to keep Senator Thorpe in her role as the Greens’ First Nations spokeswoman, despite her denunciations of the voice at an Australia Day rally.

The bombshell move changes the dynamics of the Senate and could make Senator Thorpe a key vote for the remainder of the current parliament.

The government had needed the support of all Greens MPs and one other member of the crossbench to pass legislation, either Senator David Pocock or Jacqui Lambie or her colleague Tammy Tyrrell.

“I will be able to speak freely on all issues from a sovereign perspective without being constrained by portfolios and agreed party positions,” Senator Thorpe said.

She said she would vote with the Greens on climate change issues, but seemingly foreshadowed taking a more independent tack.

“I’m not announcing my final position on the voice and today I want to continue my negotiations with the government,” she said.

Party insiders said that Senator Thorpe had clashed frequently with South Australian Senator Sarah Hanson-Young over the former’s more radical political inclinations.

Senator Thorpe was more closely aligned with Greens MPs considered to be on the party’s left, including some newly elected Queensland MPs and NSW senators David Shoebridge and Mehreen Faruqi.

“I do not intend to comment further about my time in the Greens,” she said.

A series of damaging leaks about Senator Thorpe’s behaviour emerged during the first six months of Parliament.

She resigned as the party’s deputy leader only last October after she was revealed to have been in a relationship with ex-bikie Dean Martin while she was sitting on a parliamentary committee that received confidential federal police briefings.

In November Mr Bandt was forced to confirm he had met with members of the Australian Federal Police about the issue.

“I’m not going anywhere,” Senator Thorpe had said.

She has also rejected allegations by two former Greens staffers that she had behaved in a bullying manner in a meeting with an Indigenous elder.

Senator Thorpe’s term expires in 2028.