Farmers may not have to pay back millions of pounds mistakenly handed out in single farm payment cheques by the Rural Payments Agency.
Experts claim European Commission rules mean farmers may be able to challenge RPA demands for recouping the £90m it overpaid.
Article 73 of Commission regulation 796/2004 states money does not have to be returned if an overpayment was made in error by the RPA and could not be “reasonably detected” by the farmer, or if the agency did not try to reclaim the money within 12 months.
Chartered surveyor Chris Windle said while it was not clear what the rules meant by “reasonable detection”, the legislation indicated the RPA could not insist on repayment if a farmer did not want to give the money back, or if they were unable to pay.
“If someone expected to get £20,000 and they got £20,800, could they reasonably detect that?
“Some farmers will want to pay the money back when they get a letter, but lots of them will be in financial difficulty if they do.
“Farmers may want to take advice on the RPA’s legal position before agreeing to a repayment.”
Mr Windle, of Windle Beech Winthrop, said several of his clients were contesting repayments and were waiting to hear from the RPA.
“Until someone contests the repayment and the rules are tested, we don’t know what’s going to happen,” he added.
Nina Winter, NFU chief legal adviser, said despite the regulations, farmers could only argue they should not repay overpaid sums in certain circumstances.
“It depends on the facts of each farmer’s case,” she said.
An agency spokeswoman said: “We request recovery of overpaid money as we consider this is the right action to take in the interests of equity of treatment and protecting the public purse.”
The National Audit Office’s report said it felt the agency could reclaim all the overpayments under UK law, she added.