Impending government reforms to probate fees could cost four of the top legacy-receiving charities a total of more than £1.5m a year, Third Sector has found.
The RNLI estimates that the changes to probate fees will cost it £400,000 a year, while the RSPCA and Macmillan estimate it will cost them £250,000 and £300,000 a year respectively.
CRUK has already said it expectd the reforms will cost it £600,000 annually.
The other charity in the top five legacy-receiving charities, the British Heart Foundation, was unable to provide a figure to Third Sector.
Last year, the government announced that it was revising its previous proposals for reforming probate fees, which were dropped because of the 2017 general election, and would be pushing ahead with removing the existing flat rate of £215.
Instead, it said, probate fee bands would be brought in, with estates of more than £50,000 paying between £250 and £6,000, and the maximum amount reserved for estates worth more than £2m.
The introduction of the probate reforms has been delayed owing to the time needed to discuss Brexit in parliament.
The reforms were supposed to be in place on 1 April, but have yet to complete the final stage of the parliamentary process.
Craig Fordham, director of legacies at Macmillan, said: "We estimate that the proposed changes to probate fees would result in a loss of £300,000 to our annual income, which could pay for five Macmillan nurses for a year, each providing physical and emotional support to help people with cancer live life as fully as they can.
"We are funded almost entirely by generous donations and simply cannot support the growing number of people who need us without the help of the public."
An RSPCA spokeswoman said: "The RSPCA could see a reduction of £250,000 per year as a result of these proposals, which is disappointing for a charity such as ours, which carries out vital and unique animal welfare work and relies on the generous donations left to us in the wills of animal lovers across the country."
A Ministry of Justice spokesman said that the fee changes were vital to the effective running of the courts and tribunals system.
"Our changes will not affect fixed-sum charitable donations and will see an extra 25,000 bereaved families paying no probate fees at all, allowing them to donate more if they wish to," the spokesman said.
"The probate fee will never amount to more than 0.5 per cent of the estate value and around 80 per cent of applicants will pay £750 or less."