Charity workers told Turning Point bosses today to get back round the table or face strikes over their plan to sack 2,600 people and re-employ them on worse contracts.
The workers' union Unison said it was "outraged" and had sent a "strongly worded" letter to the social care charity warning that a ballot for industrial action is on the cards unless it backs down.
The union added that to "add insult to injury" the charity has engaged in a "systematic policy of union derecognition" and accused Turning Point of a lack of consultation.
"It is untenable for Turning Point management to attempt to ignore the wishes of its staff to be represented by a union of their own choice," Unison said.
The letter warns that the union's hundreds of members in Turning Point are to be balloted for industrial action at the beginning of January in opposition to the proposed changes.
National charities officer Simon Watson said: "It is a disgrace that a charity like Turning Point set up to care for people in need should treat their own staff so badly.
"We have written to chief executive Lord Victor Adebowale warning that time is running out to avoid a ballot for industrial action.
"Even at this late stage, we are willing to meet and work together to resolve the situation."
In a statement Turning Point said: "We are in active talks and negotiations with our recognised union, Unite, with whom we have an exclusive collective bargaining agreement and who, as such, represent the whole of our workforce.
"This dialogue is underway and we are fully committed to the official recognised formal consultation process with the union.
"We are keen to promote engagement at all levels. We have been in regular contact with the recognised union and want to work with them to ensure a smooth process."