Researchers visited one MFI store at two-week intervals over a six-month period to establish the pricing levels of Sales and price promotions on five of its kitchen ranges.
The researchers reported that four of the five kitchens had never been sold at the higher price, on which a range of discount claims were based.
Which? said: 'MFI's pricing policy deceives its customers into thinking they are getting a better deal than they are. In some cases its pricing is downright misleading.'
The consumer body believes MFI 'is in danger of breaching the consumer code of practice' and has passed its findings on to Trading Standards.
MFI has rebutted the report, claiming it contains 'numerous factual inaccuracies'. It affirmed that it meets all advertising and Sales promotion legislation, and goes beyond the minimum requirements of the code.