In a speech to the National Press Club in Canberra, Senator Milne says it has become clear that Labor no longer has the "courage or the will" to work together on a shared national agenda.
"Labor has effectively ended its agreement with the Greens," Senator Milne told the audience.
"Well so be it, but we will not allow Labor's failure to uphold the spirit of our agreement to advance the interests of Tony Abbott.
"We will not walk away from the undertakings we gave not only to the Prime Minister, but to the people of Australia, and that was - to deliver confidence and supply until the Parliament rises for the election."
More to follow.