Farmers with RHI boilers that received a letter from the Office of Gas and Electricity Markets (OFGEM) must respond without delay, the Ulster Farmers' Union has said.
The OFGEM letters regard the Renewable Heat Incentive (RHI), which was Northern Ireland's botched green energy scheme.
Set up in 2012, the RHI was an attempt by the Northern Ireland Executive to help to increase consumption of heat from renewable sources.
It offered financial incentives to farms, businesses and other non-domestic consumers to use biomass boilers that mostly burned wood pellets, as well as solar thermal and heat pumps.
But flaws in setting the scheme's subsidy rate left it open to abuse as claimants could earn more cash the more fuel they burned.
The Northern Ireland Executive collapsed after deputy First Minister Martin McGuinness resigned in protest over the scandal.
Now the Ulster Farmers' Union (UFU) has said that OFGEM's letters to farmers demand them to hand back alleged overpayments, placing 'additional stress on farmers already under enormous pressure.'
According to the farming union, in some cases, tens and even hundreds of thousands of pounds in repayments are being demanded.
“The grounds for seeking to impose these sanctions are flimsy and unacceptable and so too is the time frame for repayments," said John Watt, UFU rural enterprise chairman.
"These tactics are unfair and are a mental health risk to farmers who are unknowing victims of a botched scheme.”
OFGEM stated that claim allegations had been proven, but the UFU said that in many instances this was based upon participants failing to present evidence to the contrary.
OFGEM are now seeking repayments from farmers within 28 days.
The UFU has urged farmers unhappy with OFGEM's decision to consider seeking a statutory review, which must be sought within 28 days of receiving the letter.
Mr Watt said: “The all important message however is to remember that 28 day limit to mount a defence. Inaction is not an option.”
