The Presbyterian Mutual Society went into administration in November
Twenty-three former Presbyterian moderators have written to the British and Irish governments.
They have asked them to help the Presbyterian Mutual Society, after it went into administration in November.
The moderators said this was partly caused by the government not extending its savings guarantee scheme to the society.
The Very Reverend John Dunlop has said the prime minister has a moral obligation to help.
"Whenever Gordon Brown became prime minister he said he would be guided by the ethics and morals of his father," he said.
"Now Gordon Brown was raised in a Presbyterian manse, his father was a Presbyterian minister.
"So Gordon Brown understands what moral and ethical obligations are.
"It may be that there is no technical legal obligation to help the mutual society but we believe that there is a moral obligation to help the mutual society."
Earlier this month, the PMS administrator, Arthur Boyd, wrote to 9,500 investors recommending that they accept a proposal, which he said would give them some of their money back.
He recommended an "orderly run down" of its business over time.
If this plan is to go ahead, more than 50% of investors must vote to back it by the deadline of noon on Friday.