Tesco and Sainsbury's have admitted that they use data about customers' grocery and other shopping habits when they set prices for financial services such as car insurance.
Regular shoppers - who make predictable and repeat visits - are expected to benefit most from reduced car insurance premiums.
Both supermarket giants amass huge volumes of customer data, which their financial divisions can then access when setting prices across a range of deals. But critics say the use of this type of data is "intrusive" and they question its relevance to the likelihood of a policyholder making a claim.
“Big data” is a controversial topic in the insurance industry, with companies increasingly likely to trawl through consumers' personal information looking for patterns that could indicate risk.
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Information scraped from social media sites and collected via devices such as in-car "telematics" boxes, which monitor drivers' habits and locations, could become too significant a determinant of price, some commentators have warned. This would undermine the principle of insurance, it is claimed.
Sainsbury's said it takes into account the size of customers' shops and how often they visit stores to offer a "preferential discount" on a range of insurance products.
Tesco declined to explain how data was used, but said it used details collected via its Clubcard loyalty scheme "to offer customers even better value" on products such as car, home and pet insurance.
James Daley, founder of Fairer Finance, the consumer lobby group, said consumers already had to give insurance companies personal information - and that this additional data should not be allowed to further influence premiums.
He said: "Why should a car insurer need to know when and where you shop in order to give you a quote? It's got nothing to do with how well or safely you drive."
Mr Daley said he was concerned that the use of shopping data would be the next step into a pricing model that lumps people together into "arbitrary buckets" based on statistics.
"This will end up with people being pigeon-holed and the benefits of pooling risks will be lost."
While there is no suggestion that either supermarket is breaching data rules, Simon Morrissey, head of data and privacy at law firm Lewis Silkin, voiced concern over the transparency of the use of personal data.
He said it was one thing to reward a loyal customer with a discount, but using shopping data to assess a customers' risk profile may be going beyond what most people understood to be the agreed use of their information.
"Consumers are unlikely to have read fully all the detailed smallprint about how their data might be analysed and used," he said.
Last year Andrew Bailey, chief executive of the Financial Conduct Authority, warned against big data being used by insurers to “identify customers more likely to be inert”. He was concerned this would enable firms to over-charge those unlikely to shop around.
His remarks followed controversy involving Admiral, the insurer, which announced that it planned to trial the use of Facebook status updates and “likes” to help establish which customers were safer drivers.
The trial was specifically aimed at young motorists' "first car quote" and data would only have been used with their consent.
Admiral's move was heavily criticised by campaigners who called the proposals intrusive and the social media giant then blocked Admiral’s technology hours before the service was due to launch.