Britain's largest trade union today described staff pay and conditions plans put forward by the RSPCA as "among the most aggressive set of proposals" it had ever seen.
Unite members at the animal charity have until 5 December to vote in a consultative ballot that could lead to a full-scale strike ballot.
In a 2,000-word open letter to Chris Sherwood, chief executive of the RSPCA, Unite staff at the charity describe recent talks with management, which broke down, as "deliberately restrictive, disingenuous and designed to fail".
The letter adds: "When the union’s negotiating team showed the society’s proposals to Unite’s legal team in London, their response was ‘these rank among the most aggressive set of proposals ever seen by Unite’ and they had never seen such a document within the charitable sector before.
"It was clear to all the proposals were not just financial but ideological, with a deliberate intent to remove the union from all future negotiations and diminish the working conditions for all staff, despite the assurances from the society."
The letter, which is signed by "all of us at RSPCA branch of Unite the union", says plans to replace incremental pay with performance-related pay "could exacerbate plummeting staff morale in an organisation where bullying has been endemic".
It also claims the charity's respect policy is "not fit for purpose" and some senior managers had made "veiled threats" to union representatives.
"The union has been approached by many from middle and senior management to quietly and confidentially voice their concerns," the letter says.
"They won’t raise their worries during meetings or management phone hangouts because they are simply too scared to do so. They fear for their jobs.
"This organisation has a culture of bullying and harassment, and that originates from the very top, from some of the people you have employed around you. Sadly, you appear not to see this or be able to do anything about it."
Sherwood, the letter claims, is "surrounded by a number of external senior managers who are employed on a short-term basis with a clear and cold agenda".
The letter says Unite members make up the majority of the RSPCA's 1,700 staff. A union spokesman would not reveal the precise number when asked by Third Sector.
The union alleges its consultation with management effectively left only nine days for negotiation and agreement and urges Sherwood to establish another 45-day consultation for "genuine talks".
The letter warns: "If the executive continues with its current strategy and keeps to its path of enforced contractual change, the weakening of the union and the erosion of workplace policies, then industrial action may be an option we will be forced to explore."
Jesika Parmar, a Unite regional officer in south-east England, said: "This open letter is a devastating critique that shows a management that has walked into a cul-de-sac of its own making. Now is the time for a managerial U-turn and for genuine negotiations to take place."
The RSPCA generated income of £142m in the year to 31 December 2018, up from £140.9m the year before, but spent £159.8m. This is the first time expenditure has outstripped income in at least the past six years.
An RSPCA spokeswoman said it faced "very challenging financial times and we have to act now to bring our costs in line with our income in order to be sustainable into the future."
It plans to finalise new arrangements by spring next year before phasing in whatever is agreed.
The spokeswoman added that the RSPCA did not recognise the union depiction of negotiations.
"Our door is always open to the union and we hope it will choose to come back to the table," she said.
"The RSPCA entered into talks with a genuine desire to reach an agreement to ensure the RSPCA has a financially sustainable future, so we can continue helping animals most in need.
"Throughout this process employees have been reassured that the current base pay is not affected and the proposals regard only allowances and increments.
"Performance management of annual increments is usual in most modern workplaces, would be developed with the union and employees, and would not be fully introduced until 2022."
The spokeswoman refuted Unite's claim that staff morale was plummeting. She said it had listened to feedback from the union and staff and made changes based on that.
She added: "The RSPCA has consistently stressed that it has no intention of making any changes to employee policies such as sick pay and does not wish for protracted discussions because this would prolong uncertainty and concern for employees.
"We recently launched our respect policy, which lays out the RSPCA’s commitment to an inclusive and supportive working environment that embraces diversity and inclusion and, importantly, our zero-tolerance approach to bullying and harassment."