Last month, the retailer took the own label versus brand war to the next level when it launched an eight-day Taste Test Challenge.
More than 650,000 shoppers in 400 stores compared Sainsbury's own label Tip & Mix yoghurt, ice cream, classic lemonade, high juice, strawberry light yoghurt and tomato ketchup with brands such as Müller, Wall's, Ribena and Heinz.
This week, Sainsbury's revealed the results, which found the majority of shoppers favoured the own-label lines, with the exception of the strawberry light yoghurt. Own-label tomato ketchup and classic lemonade scored the highest, with 61% of the votes.
But one supplier told The Grocer it had carried out its own mystery shops to monitor the taste tests and accused Sainsbury's of using hard selling and heavy-handed tactics to get the results it wanted.
"The staff were pushing shoppers to vote for the own-label product even when they had chosen the branded equivalent," he said. "They made misleading statements to big up the qualities of the own-label product, but failed to mention the qualities that were just as good if not better in the branded equivalents."
Another supplier also disputed the results, saying his brand regularly delivered better results than its competitors in independent taste tests.
A Sainsbury's spokeswoman replied: "we have been open and transparent about the results throughout the tastings."