Raising the National Insurance rate for employers could “break us”, a charity for children with special educational needs and disabilities (Send) has said.
Family Voice Peterborough (FVP) is calling for an exemption for charities if the measure is introduced in the autumn budget.
An increase to how much employers must contribute towards national insurance is expected to be announced in the budget, to boost funding for public services.
The government says its tax regime for charities is among the most generous in the world.
Ms Ravenscroft said her charity could lose staff and be forced to scale back its services.
“If there’s no protection for us as a charity, it could break us, because we’re only just coping at the moment,” she said.
“We’ve had to downsize the charity due to the cost of living, which is still affecting us.
“Our costs are rising, while at the same time the need for our services is also increasing.”
The charity has already given up its former base, the Goldhay Centre in Peterborough, due to rising costs.
A HM Treasury spokesperson said: “The UK is a world leader in the charitable sector. Our tax regime for charities, including exemptions from paying business rates, is among the most generous of anywhere in the world.
“We do not comment on speculation around tax changes outside of fiscal events.”
Ms Ravenscroft said a potential rise in fuel duty would also increase its costs, as the charity covers some travel expenses for its employees.
“There’s no talk about any protection for charities in any of this,” she said.
FVP trustee Mandi Rennie said her 17-year-old son, who has ADHD, is concerned about its future.
“I think he’s worrying about where he is going to go for advice if the service does get cut,” she said.
“He can always ask me but some of the questions are things he doesn’t want to ask me because I’m too close to him. He’d rather ask someone else; someone more detached.”
Details of the budget will be revealed at 12:30 GMT.