Insurers are trying to force drivers to settle claims just hours after accidents, says a road safety charity.
Brake is concerned that people are being encouraged to take a pay-out before getting legal or medical advice.
Solicitor groups have called for tighter rules governing how companies treat so-called "third parties".
But the insurance industry says such claimants should get compensation quickly without having to resort to lengthy and costly legal processes.
When a driver is involved in an accident which is not their fault, they would expect to deal with the other person's insurer.
But the national road safety charity, Brake, says it is concerned that insurance companies are pushing crash victims to settle quickly, before getting legal or medical advice.
Kimberley Harrison suffered severe facial injuries when another car crashed head-on with hers in March 2008.
She was surprised - and angry - to receive frequent calls from an agent of the other driver's insurer the day after she left hospital.
"From the day I got home, the insurance company phoned me and were pressurising me not to take it any further - not to seek legal advice. I was really shocked.
"He was really forceful, like a bully - really trying to push me to close a deal," she said.
Once she instructed lawyers, Kimberley said the insurance company in question, Quinn Direct, managed to get hold of her medical reports.
"They posed as someone working for my solicitor in order to obtain my medical records. I had no idea insurance companies would behave in that way."
Brake spokesperson Jane Horton said insurers should not make this kind of direct and often unsolicited contact.
"It's as if having been made a victim once… you're then being made a victim twice by then being approached when you're not really equipped to deal with it," she added.
Tommy Scott is a former claims handler for Quinn Direct. He said it was his job to "doorstep" third parties, often within hours of the accident.
"My sole job was to capture those clients - to stop them getting independent legal advice, and try to settle direct in their living room," he said.
Quinn Direct has denied Tommy Scott's claims.
It said its "pro-active" approach is "based on paying fair compensation" quickly, and that third parties "appreciate" the service.
The company added that it "completely respects a claimant's right to appoint a solicitor".
Quinn Direct is investigating the claims about Kimberley Harrison's medical reports.
Personal injury lawyers say that the practice of "third party capture" is widespread across the industry, and that most big insurers have departments dedicated to it. Direct contact with the not-at-fault driver is standard practice.
A leading insurers association (the ABI) insists its members contact injured third party drivers to help them get compensation quickly.
"It is the right thing for insurers to be doing, rather than requiring claimants to drag them through the courts," said Justin Jacobs, assistant director of motor insurance at the ABI.
But the Motor Accident Solicitors' Society and Brake are both calling for tighter regulation in this field.
The rules which govern how insurers must behave focus on insurance companies' treatment of their policyholders; they say very little about how third party claimants should be treated.
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