In a letter sent with voting papers, the RSPCA's council chairman Michael Tomlinson says the charity is "at risk of infiltration from people who may not have the best interests of animal welfare and the RSPCA at heart".
Members have until 18 June to vote for five new members of the RSPCA's 25-strong ruling council.
But four of the 12 candidates are understood to be hunt supporters and members of established hunts.
Third Sector understands that the RSPCA is looking to establish whether the pro-hunting candidates, who all joined the charity in 1996, did so for "ulterior
motives. At present it cannot prove that they are part of a concerted campaign to change the charity's policy on blood sports.
In January 2001, the RSPCA won a High Court battle to expel members who join for "ulterior
motives rather than to further the aims of the charity to combat cruelty to animals. Last June, the charity expelled former Olympic showjumper Richard Meade for running a campaign to infiltrate the society with members of the British Field Sports Society, now the Countryside Alliance.
In 1996, Meade wrote to members of the British Field Sports Society encouraging them to join the RSPCA to overturn its opposition to hunting. It is estimated that 5,000 members of the Countryside Animal Welfare Group (CAWG) - the name of the campaign to infiltrate the RSPCA - subsequently joined the charity.
RSPCA rules require that only those who have been members of the charity for five years can stand for election to its ruling council. This year is the first time that members of CAWG influx can legally attempt to get elected.
A spokesperson for the RSPCA said the charity was a broad church and included both anti- and pro-hunting members, but it reserved the right to exclude members who had joined solely to further an agenda inconsistent with the charity's aims.
Joe Saxton, RSPCA council member from 1995 to 2001, was scathing about the latest attempt at infiltrating the charity by hunt supporters. "This is a sad attempt by a desperate and deranged group to change the RSPCA's policy on hunting,
he said. "Even if all four of this group were elected, which is highly unlikely, it would take another four years for them to take over. By that stage, the debate on hunting will be history."