US researchers have discovered a likely reason why people find it hard to do two things at once.
One in five people admit to talking on the mobile phone while driving
A "bottleneck" occurs in the brain when people attempt to carry out two simultaneous tasks, the research shows.
The study found the brain slows down when attempting a second task less than 300 milliseconds after the first.
The findings, published in Neuron, support the case for a complete ban on the use of mobile phones when driving, the team said.
Participants were asked to press an appropriate computer key in response to one of eight different sounds and call out a syllable in response to eight different images.
The researchers from Vanderbilt University used functional MRI scans to detect changes in oxygenated blood in the brain - a way of monitoring the activity in different brain regions.
They found that the lateral frontal and prefrontal cortex, and also the superior frontal cortex, were unable to process two tasks at once, leading to a bottleneck.
But when tasks were presented a second apart there was no delay.
Study leader Dr Paul Dux said previous studies had shown people were limited in being able to do two simple tasks at once - a phenomenon known as "dual-task interference".
"We were interested in trying to understand these limitations and in finding where in the brain this bottleneck might be taking place," he said.
"We determined these brain regions responded to tasks irrespective of the senses involved, they were engaged in selecting the appropriate response, and, most importantly, they showed 'queuing' of neural activity.
"The neural response to the second task was postponed until the response to the first was completed," he added.
The researchers said the study was particularly relevant to tasks that people have to do in a complex environment, such as flying a plane.
Talking on mobile phones when driving was also dangerous, they said because motorists are bombarded with visual information and might also be talking to passengers.
"Our new research offers neurological evidence that the brain cannot effectively do two things at once. People think if they are using a headset with their cell phone while driving they are safe, but they're not because they are still doing two cognitively demanding tasks at once," said co-author, Dr Rene Marois
Dr Dux added "Dual-task costs can be up to a second, and that's a long time when you're travelling at 60 miles per hour."
The law banning use of mobile phones while driving is to be strengthened from the 27th February when drivers who are caught will get three points on their licence as well as a £60 fine.
A spokesperson for the Royal Society for the Prevention of Accidents said the research confirmed what other studies had shown.
"The research we have seen is whether your on your mobile phone or a hands free kit, you're four times more likely to have a crash.
"It is why we keep saying to people, that the safe thing to do is to turn your mobile off when you are in the car.
"Generally we need people to take on board this research and understand just how dangerous it is."
Dr Narender Ramnani, reader in cognitive neuroscience at Royal Holloway University of London said: "We've always known that doing two things at once is difficult but what MRI tells us is where that bottleneck is occurring.
"We tend to overestimate our ability to do two things at once.
"It highlights that even areas such as the prefrontal cortex, which are very sophisticated, can only do one thing at a time."