Unlike the big four, Waitrose has increased the prices of many branded lines across a wide range of categories since March, according to The Grocer 33 pricing database, including frozen, confectionery, household, dry grocery and dairy products - despite its previous tough negotiation tactics, which it claimed were necessary to compete with rivals.
Earlier this year, the retailer wrote to both branded and own label suppliers requesting cuts of "at least 2% off cost prices", in a move that was compared to M&S's Project Genesis.
Of the brands that have had price hikes, Birds Eye Chicken Dippers showed the biggest jump, with a 513g bag up 25p between 24 March and 14 April.
Other increases include Old El Paso Fajita Dinner Kit, 505g, which rose 21p to £3.20 and Mr Muscle Bathroom Cleaner 500ml, up 10p in the same period. There were also rises on Marmite 125g by 9p, straight cut McCain Home Fries oven chips 1.5kg by 20p and Batchelors Super Noodles 100g by 4p, between 17 March and 14 April. None of the other leading multiples increased the prices of these brands during the period.
One supplier claimed the retailer had bumped up more than a quarter of his lines - some by as much as 7%, yet like others, he had been kept in the dark as to why. " Waitrose is making a big mistake - uncompetitive pricing is a poor strategy, especially in this economic climate," he added.
Many believe the move is designed to recover the £25m the retailer is ploughing into its 1,400-strong Waitrose Essential range, although it has yet to be fully rolled out.
The retailer denied any link between the price rises and the Essential launch.
"Although prices may occasionally vary, we are committed to long-term competitive pricing," said a Waitrose spokeswoman.