More than 20,000 people have signed a petition started by RNLI supporter Ian Smith on www.number10.gov.uk, the official site of the Prime Minister's Office.
The communications regulator has proposed a new pricing structure that the RNLI has estimated would increase its annual fees from £38,000 to £260,000.
Ofcom responded to the petition by saying it planned to change the way the fees were calculated, which might mean the RNLI paying only £20,000 a year.
Peter Bradley, operations manager at the RNLI, described Ofcom's response as "woolly". He added: "We can't turn the heat off just because they are making a suggestion that might not happen.
"We don't pay fees for boats, so a precedent is being set. We provide a valuable service - if we didn't, the Government would have to."
An Ofcom spokeswoman said it was willing to listen to the RNLI.