Drivers are four times more likely to crash when using mobile phones, even if they use hands-free kits, experts say.
It is illegal to use a hand-held phone while driving in the UK
They reached their estimates by looking at the phone bill records of 456 drivers needing hospital treatment after road crashes in Perth, Australia.
In the UK it is illegal to use a hand-held mobile phone while driving.
Safety campaigners say the University of Western Australia study in the British Medical Journal shows the rules should apply to hands-free phone use.
For each driver, the researchers assessed phone use immediately before a crash and on trips at the same time of day 24 hours, three days, and seven days before the crash for comparison.
Mobile phone use in the 10 minutes before a crash was associated with a four-fold increased likelihood of crashing.
This was irrespective of whether the driver was using a hand-held or hands-free phone.
Similar results were found for the interval up to five minutes before a crash.
Author Suzanne McEvoy and colleagues from the University of Western Australia said: "More and more new vehicles are being equipped with hands-free phone technology.
"Although this may lead to fewer hand-held phones used while driving in the future, our research indicates that this may not eliminate the risk.
"Indeed, if this new technology increases mobile phone use in cars, it could contribute to even more crashes."
A spokesman from the Royal Society for the Prevention of Accidents said: "This is exactly what we have said and have known for some time.
"We hope that the people who callously think that their phone call is more important than somebody's life will get the message eventually when they see more and more research like this."
He said the current ban on using hand-held mobiles while driving, which can carry the penalty of a fine and in the future possibly also up to three points on the driver's licence, should be extended to hands-free phones.
However, the study authors said this would be difficult to enforce.
They said a possible solution might be to change mobile phones so that they cannot be used when vehicles are in motion, but added that industry was unlikely to embrace this.