The RSPCA has announced that it will be cutting jobs in a restructure after it experienced what it calls a "net cash outflow" of £6.1m last year.
The charity said it would have to prioritise its front-line animal welfare work in the face of rising costs and a fall in legacy income.
Mike Tomlinson, the charity’s chair, wrote to staff warning them that the organisation would be restructuring in a bid to make savings of £4.5m a year.
He said the charity would carry out a 45-day consultation with staff about changes to the organisation.
He did not say how many jobs would be lost among the RSPCA’s 1,568-strong workforce.
Tomlinson said in a statement that the charity had already made significant budget cuts in the past five years. "However, last year’s figures, when the charity had a net cash outflow from its core work of £6.1m, are clearly unsustainable and show that things cannot carry on as they are," he said.
"Given rising costs, including private boarding as well as fuel, energy and veterinary bills, our operational costs are increasing faster than income is being generated. We have already started to implement plans to diversify the society’s income into new areas such as events and business, which will see the RSPCA move away from a reliance on legacy income.
"However, these are long-term plans and the RSPCA has to address the reduction in our income in the short term."
The charity, which had an income of £132.8m in 2012, said that complaints about animal cruelty were increasing and it had to spend £4.5m last year on emergency boarding costs for animals because there was no suitable space in its centres.
Sources in the organisation said they were shocked by the letter. "We knew there was a problem, but the situation is a wholesale crisis," said one. "I don’t think people had realised the size of the problem until now."
The charity currently has no chief executive because Gavin Grant stood down on health grounds after two years in charge.
The RSPCA said at the time that John Grounds, its director of marketing and campaigns, would represent the charity externally and cover its public commitments. But only six weeks later Grounds left the charity with immediate effect.
The charity was heavily criticised in sections of the right-wing press in 2012 after it spent more than £320,000 pursuing a successful prosecution of the Heythrop Hunt in Oxfordshire.
It investigated 150,000 incidents of alleged animal cruelty in England and Wales in 2012, leading to 2,000 cases appearing before the courts and 4,000 convictions.