In a message to union members at the charity, Unite said there had been an "inevitable breakdown" in discussions about the charity’s plans to introduce performance-related pay for salary increments for all of the RSPCA’s 1,700 staff members.
The charity said it was disappointed negotiations with the union "appear to have reached an impasse", but said it would continue dialogue with staff.
Third Sector understands staff will be dismissed from the charity if they do not sign the new contracts by 31 March next year.
Unite has previously accused the RSPCA of deliberately setting up negotiations over the issue to fail by not allowing enough time or providing enough information, and repeated the allegation in the message to union members after a final meeting on 12 November.
The latest message said the charity had provided detailed information slowly "so as to prevent any serious or constructive engagement" and information provided at the meeting on 12 November should have been provided at the beginning of talks on 3 October.
"This has resulted in the unfortunate but inevitable breakdown in these discussions," the message said.
It said the union had asked for more time and "proposed a number of options which could have led to potential agreement in some key areas", but the RSPCA had refused and had not demonstrated any flexibility on the proposals.
"It is Unite’s opinion the whole process has been deliberately created to not be meaningful in any sense of the word," the message said.
The union has also previously express concern about bullying at the charity. The charity has denied this has occurred.
In the message to union members, Unite said: "Unfortunately, our negotiating team, which has experience of many previous negotiations, has found the whole process unpleasant, oppressive and in conflict with the society’s own claims it is opposed to bullying.
"There exists the feeling that we, and by extension our membership, have been treated with contempt."
The message did not say what the union plans to do next, but does contain an information panel on industrial action, which it said was a "last resort".
It said there were other ways apart from going on strike for employees to demonstrate unhappiness and that any potential action would not affect animal welfare.
A spokeswoman for the RSPCA said: "We’re extremely disappointed that negotiations with the union appear to have reached an impasse, but we will continue engaging with our employees, most of whom are not union members.
"Employees have been reassured that basic pay is not affected, there is no proposal to freeze pay and there are no plans whatsoever to dilute any of our existing policies."
"The RSPCA entered into talks with Unite in good faith and with a genuine desire to reach an agreement, and we remain committed to reaching a conclusion that is fair and affordable."
She said the charity was facing "very challenging financial times" and had to act to bring its costs in line with income.
"The society has made it clear that we are still open to proposals and suggestions from the union," she said. "In the meantime, we will continue to keep our employees updated about our plans."