Mobile phones may improve memory and protect against Alzheimer's disease, scientists have discovered.

In one of the most unexpected scientific findings for some time, researchers have found that the electromagnetic waves emitted by the devices may improve cognitive function.

After years of health warnings about mobile phones, scientists in Florida admit they were as surprised as anyone when their research showed they might be good for the brain.

But they have enough confidence in their results to recommend that the electromagnetic waves the phones emit should be "vigorously investigated" as a memory enhancer and treatment for Alzheimer's.

Mobile phones have been suspected of causing problems ranging from ear ache to brain cancer by raising the temperature of the head and exposing cells to "oxidative stress".

Inquiries into their safety have been held, but no conclusive evidence of damage has been found.

Scientists in the Department of Cell Biology at the University of South Florida attempted to quantify the damaging effects by exposing mice to levels of radiation similar to that emitted by mobile phones.

The mice received two one-hour doses, delivered in the morning and afternoon each day, for up to eight months - equivalent to that of a frequent mobile phone user.

Half of the mice were genetically engineered to have symptoms of Alzheimer's disease. In them, exposure to the electromagnetic waves was associated with disappearance of the "amyloid plaques" - protein deposits in the brain - believed to be a cause of dementia. In the younger mice, the electromagnetic waves prevented amyloid plaques building up and had "beneficial cognitive effects".

The researchers, writing in the Journal of Alzheimer's Disease, suggest the waves may prevent plaques sticking together, forming clumps, or may stimulate the neurons.

Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "This research has been carried out in mice that mimic some of the symptoms of Alzheimer's in people, so we don't know if any similar effects will be seen in humans.

"Although the researchers hope their findings will translate to people, much more research is needed to find out if there could be any beneficial effects of long-term exposure to electromagnetism, and to guarantee its safety. We don't recommend spending 24 hours a day on a mobile phone - we don't know the long-term effects, and bills could go through the roof."