There is no firm evidence that low-level radiowave radiation from phones can cause damage to the brain.
Some of the UK's estimated 30m mobile users have been using hands-free kits so they do not have to place the body of the phone next to the head.
But two studies commissioned by the Consumer's Association suggest that, far from reducing the amount of radiation reaching the brain, hands-free kits may actually increase it under some circumstances.
It has been suggested that the wire connecting the earpiece to the phone acts as an aerial and channels radiation towards the head.
The first study, published in April, was contradicted by Australian scientists who used different tests to measure how much radiation was being absorbed by the brain. They found the kits reduced the amount absorbed by more than 70%.
However, the Consumer's Association on Thursday returned with further study results which back up their earlier findings.
Straight wire
And they suggested that the Australian research might be misleading, as the wire was not allowed to hang straight down during the tests.
When the wire was straight, they found radiation passing into the brain was much higher.
Helen Parker, Editor of Which?, said: "It's clear that consumers can't rely on hands-free kits to reduce radiation emissions at the brain from mobile phones.
However, the research team found that attaching a metallic compound to the connecting wire just below the ear seemed to reduce the radiation.
Ms Parker said: "We're looking to the hands-free kit manufacturers to carry out further research and - if they're proven to work - incorporate ferrites into hands-free kits."
A spokesman for the Federation of the British Electronics Industry insisted, however, that all mobile phones in use in the UK had passed international safety guidelines.
A Department of Trade and Industry spokesman said the CA's report was the first to throw doubt on the earlier government tests.
"We do have material reservations about the methodology the Consumers Association have used for their tests," she said.
"We are going to be working with them to establish how the differences in our methodologies arose.
"It is important that we now need to develop quickly an agreed and transparent standard for measuring the SAR rate in hands-free kits."
The survey included phones and kits from Ericsson, Nokia, Panasonic and Philips.