BBC News' Pallab Ghosh reports on the findings

The National Radiological Protection Board (NRPB), which advises the government on safety levels, said the study "lacks statistical precision" to draw such conclusions.

The findings were revealed on Monday in an edition of the BBC's Panorama programme.

The researchers who conducted the study called on the mobile phone industry to acknowledge that its product poses a risk to human health.

But the industry denied there is any proven risk to health associated with the devices.

Reduced time using phones

Scientists told Panorama that people should cut down the time they spend using mobile phones, and consider using low-emission phones or hands-free devices.

Research commissioned by the programme showed large differences between the amount of microwave radiation absorbed by the brain from different makes of phone.

The scientists say the findings from the new studies could be a "problem", and are calling for the public to be given full health information on the use of mobiles.

Other investigations have recently suggested that it is too soon to say for certain that mobile phones cause harm to health, and one even found they can help to improve reaction times.

'Biological indication of problems'

But on Panorama, Swedish cancer specialist Dr Lennart Hardell, author of one of the new studies, says there is a biological indication of a problem that needs further research.

Panorama's Paul Kenyon: Brain tumours three times more likely with mobile phones

"I think that until we have the definite conclusion, the definitive results of much larger studies, we need to minimise exposure to human beings," he says.

His study, which has yet to be published, looked at brain tumour sufferers and found a connection between phone use and cancer.

Paul Kenyon on a former BT engineer's ordeal

He found that for those using their mobile phone on the right side of their head, the risk of getting a tumour increased by almost two-and-a-half times.

The risk for those using their phone on the left side was also increased by almost two-and-a-half-times.

'No overall increased risk'

But the NRPB said it has seen the Swedish findings, and quotes the authors as saying: "In this study we did not find an overall increased risk for brain tumour associated with exposure to cellular phones."

The NRPB said the study did not involve enough people to offer compelling evidence, and any difference in risk it did find was not statistically significant.

It said that the study showed there was no evidence of increased risk with increased use.

Information on the use of mobile phones was gathered by questionnaire, the NRPB said.

Scientists usually regard information gathered in such a way as being potentially unreliable.

Research funded by phone industry

However, the NRPB did not comment on another study by Dr George Carlo, head of a $25m research body funded by the mobile phone industry in the US

He speaks out for the first time on Panorama about his study, which he also says shows an increased risk of getting a type of rare brain tumour from using mobile phones.

Some people say mobile phones have made them ill

Dr Carlo's study is also yet to be published.

He said that taking into account the two new studies, it is no longer a responsible position for the manufacturers to say there is no problem.

"We clearly have results that suggest there could be something more here than meets the eye.

"The science we have today clearly shows that this is not black and white.

"That we have moved now into a grey area that suggests that there could be a problem that needs to be looked at very, very carefully.

"That grey area needs to be acknowledged," he says.

'Research indicates no health risk'

But Tom Wills-Sandford, director of the Federation of the Electronic Industry - which represents mobile phone network operators - said research supported the industry's position.

Mobile industry researcher Dr George Carlo speaks about his research

He said: "I have not yet seen the programme but if you look at the totality of evidence for any link between mobile phones and any human health effects there is no cause for concern"

The programme also spoke to people who claim they have been made ill by mobile phones.

Steve Corney, who was a BT engineer four years ago and used a new digital phone for up to five hours at a time, tells the programme he now suffers memory loss and speech problems, and is out of work.

Variation in emission rates

Research carried out exclusively for Panorama by the National Physical Laboratory also features in the programme.

Dr Hardell: "We are in a grey area still"

The study looked at the levels of emissions absorbed by the brain from different makes of mobile phone.

This is measured by what is known as a specific absorption rate (SAR). An SAR is measured by Watts of radiation energy per kilogram of brain.

An SAR of 10 Watts per kilogram is the safety limit set by the NRPB.