Egremont said the money should be diverted towards improved training and other incentives designed to boost employee morale.
The work done by undercover customers could not be verified for accuracy - which meant results were of "dubious" value, said Sue Grist, director at the consultancy. "Staff usually spot them which means employee behaviour might change in the short-term but won't be sustainable," she said. "UK supermarkets spend up to £1m a year on mystery shoppers but want to cut costs. It's surprising retailers don't just draw a red line through the expense."
Egremont believed retail executives were "hooked" on mystery shoppers in the mistaken belief that it gave them control. Mystery shoppers could often be detected by their note-taking in store and small purchases, said Grist.
However, an Asda spokesman said: "Any money we spend on this is well spent. We can monitor store performance in a way that isn't coloured by our managers who might not want to acknowledge there's an issue."
A Sainsbury's spokeswoman added: "Mystery shoppers give us a unique insight into the Sainsbury's shopping experience from our customers' eyes."