European research has shown radiation from mobile phones can damage human DNA, reigniting the debate over whether they can harm human health.
Mobile phone users are advised to keep calls as short as possible
Are mobile phones safe?
Nobody really knows the answer to this question and it has been the subject of fierce debate for many years.
There is currently no scientific evidence to suggest that mobile phones pose a risk to health.
However, since mobile phones are a relatively new technology, that situation could change.
There have been no studies into the long-term effects of using handsets, not least because the technology has not been around long enough.
What are the possible risks?
There are fears that the electromagnetic radiation emitted from mobile phone handsets may harm health.
In particular, there have been claims that it could affect the body's cells, brain or immune system and increase the risk of developing a range of diseases from cancer to Alzheimer's.
This latest laboratory-based European research, the Reflex study. suggests radiowaves from mobiles permanently alter DNA in human and animal cells.
Such mutations are seen as a possible cause of cancer.
But the research did not go on to look at whether these cellular changes were linked to disease.
Other research has also shown similar biological changes, but again, no direct evidence of harm to human health.
However, laboratory tests on mice have shown that radiation from mobile phones can have an adverse effect on their overall health.
It is not clear whether those findings can be applied directly to humans.
A study by scientists in Finland, published in 2002, suggested that the electromagnetic radiation did affect human brain tissue.
But they played down their findings saying more research was needed to see if the effects were the same in living people.
Another study by scientists in Sweden, also published in 2002, claimed to have found a link between analogue mobile phones and brain tumours.
It suggested users of "first generation" phones had a 30% higher risk of developing tumours than people who did not.
However, the findings were controversial and there have been no similar studies into the effects of modern GSM phones.
There have also been reports of people suffering from headaches, fatigue and loss of concentration after using their mobile phones.
However, these claims have not been scientifically substantiated.
What is the UK government's view?
The UK government commissioned a major study into the safety of mobile phones in the late 1990s. The Independent Expert Group on Mobile Phones published its report in 2000.
The Stewart report, named after the group's chairman Sir William Stewart, concluded there were no health risks associated with using mobile phones.
In 2002, the government and the mobile phone industry pledged £7.4m to a new research programme into the safety of mobile phones.
Its 15 studies, being carried out by independent scientists, will aim to come up with definitive answers on mobile phone safety.
The latest review of the evidence, carried out by UK government scientists, suggests mobile phones do not harm health.
However, in a report published in January 2004 members of the Advisory Group on Non-Ionising Radiation warned that the situation could change if new evidence emerges.
Are there any risks for children?
The Stewart report recommended that children should only use mobile phones in emergencies.
The recommendation was based on the theory that children could be more at risk from the radiowaves emitted by mobile phones.
This is because their brains are still developing and their skulls are thinner, making it easier for the radiowaves to penetrate them.
According to the Advisory Group on Non-Ionising Radiation, "little has been published specifically on childhood exposures" since 2000.
As a result, children are still advised only to use mobile phones in emergencies.
However, surveys suggest that many children are ignoring the advice.
A survey of 1,000 British children, carried out in 2001, found 90% of under-16s own a mobile and one in 10 spends more than 45 minutes a day using it.
Do manufacturers think phones are safe?
The mobile phone industry has spent millions of pounds investigating the safety of handsets.
It is adamant that there is no link between the use of mobile phones and any adverse human health effects.
What steps can people take to reduce any risk?
Experts suggest that mobile phone users can take a number of precautions to reduce any possible health risks.
Almost all agree that the best step is to keep mobile phone conversations short and to a minimum.
Many believe that so-called hands free kits reduce the risks by cutting the amount of electromagnetic radiation entering the brain.
They also recommend that users buy phones with external aerials so that it is as far away from the head as possible when in use.
Similarly, it is believed that phones with a long talk time are more efficient and produce less powerful emissions.
Users are also advised against buying handsets with a high "SAR" value, which means it emits more radiation.