Individuals have been urged to take care over the effect on their credit rating when searching for loans.
High Street bank TSB said some loan providers make a "hard mark" on credit files when someone asks for a loan price or quote.
Personalised pricing means that, in effect, these people are seen to be applying for a loan.
A series of these marks on somebody's file may be regarded as a danger sign by other lenders.
They might consider the person to be desperate for a loan, so more of a risk to lend to.
TSB chief executive Paul Pester said: "We estimate that consumers are losing out by as much as £400m each year, which is going straight into the pockets of aggressive loans providers. It is time the industry comes clean on these costly underhand tactics."
In a report, TSB argued: "Personal loans providers must agree never to perform a hard credit check until the customer chooses to actually purchase a loan in full knowledge of the price being offered.
"The practice whereby some providers issue a hard credit check in order to issue a customer with a personalised price needs to end. There is no reason to punish customers in this way.
"Instead, providers must commit to not performing hard credit checks until after a consumer has been given their personalised interest rate and has decided to apply for the loan. This will end an environment where some providers are effectively punishing customers for shopping around for a better deal."
James Jones, of credit report agency Experian, said individuals needed to take care.
"It is important to shop around for credit to make sure you are getting a good deal," he said.
"It is true that applying for credit is noted on your credit report by what is often called a hard footprint, which can affect credit scores. But if you are only seeking quotations you should make this clear to lenders because quotations should only leave harmless soft footprints."