Finance firm SubMe is offering loans of up to £5,000 to site workers.
SubMe is part of the OBK Group which also includes payroll companies Sprite and Boss Projects and accountancy firm OBK Tax.
Brian Rye, National Secretary of UCATT, said: “Construction workers need to be very cautious before taking out these loans.
“They might look like a quick and easy way to get credit but you will end up paying back far more than you borrow.”
The loans are taken over a maximum period of three months with a typical APR interest rate of 383%.
Malcolm Oliver, senior partner OBK Group, said: “We are committed to be a responsible and ethical lender.
“Compared to our competitors, who provide similar offerings, we are proud to be FCA regulated.
“Our compliance and ethics are at the heart of what we do and I would also like to stress that all products across the OBK Group are compliant with FCA and HMRC rules and regulations
“SubMe Ltd has strict affordability rules and we do not lend to anyone who we believe cannot afford to pay the loan back.”