The union described management at the charity as "corporate renegades" and accused them of bullying staff.
The dispute centres on Turning Point’s proposal, reported in November, to sack its 2,300 workers and re-employ them on new contracts.
The union said this would result in lower pay and inferior conditions for staff. Among the proposed changes are cuts to overtime and pay for unsociable hours, and a refusal to implement future national pay awards, it said.
The pay for some staff could be cut by up to £10,000 a year, the union said.
Unite, which has 500 members at the charity, said it had offered concessions but that talks with management had broken down.
"This is a perverse decision – a charity robbing its staff to prop up profits and boost expansion," said Jamie Major, regional officer at Unite. "It seems that corporate greed is not exclusive to the bankers and we now have our own corporate renegade right here in the charity sector.
"We will treat Turning Point accordingly by standing shoulder-to-shoulder with staff and fighting this with every tool at our disposal – nothing is ruled out."
Turning Point said it wanted to continue having talks with Unite and that the economic climate had forced it to review its costs in an attempt to protect jobs and its services for vulnerable people.
"These proposals are not being made lightly, but are forced out of economic necessity," a spokeswoman for the charity said. "The proposals are to protect as many jobs as possible by reviewing changes to our terms and conditions of employment.
"We need to move towards a market rate for employees, one that protects their base pay, and we are proposing to increase base pay for those who are the lowest paid."