Many studies have been carried out in Europe and elsewhere to investigate the possibility of links between mobile phones and various health problems.
Further research is currently in progress. Examples include the COSMOS and INTERPHONE studies.
In the COSMOS study, scientists from the UK, Denmark, Sweden, Finland and the Netherlands are monitoring almost 300,000 mobile phone users in Europe to identify possible health problems linked to the use of mobile phones over a long period of time.
The UK part of the study, run by Imperial College London, will follow the health of more than 100,000 adult mobile phone users for 20 to 30 years.
Scientists will look at any changes in the frequency of specific symptoms over time, such as headaches and sleep disorders, as well as the risks of cancers, benign tumours, and neurological and cerebrovascular disease.
The study in the UK is jointly funded by industry and government under the Research Initiative on Health and Mobile Telecommunications (RIHMT), and is managed through the Department of Health and Social Care's Policy Research Programme.
The INTERPHONE study (PDF, 176kb) was set up in 2000 and collected data in 13 countries.
The aim was to see whether mobile phone use is associated with an increased risk of head and neck tumours.
In May 2010 the results were released and indicated there was no increased risk of such tumours with mobile phone use.
But it was noted that the potential effect of long-term heavy use of mobile phones needed further investigation.
The Mobile Telecommunications and Health Research Programme (MTHR) released 2 reports, 1 in September 2007 and 1 in February 2014 (completed in 2012), which pulled together the evidence gathered in a large programme of research.
The reports published by the MTHR found no evidence of risks to health from the radio waves produced by mobile phones.
But it was acknowledged that possible effects from long-term use could not yet be ruled out and further research was recommended.
The Advisory Group on Non-ionising Radiation (AGNIR) has also carried out reviews of the potential health effects of radio waves, the most recent of which was published in 2012.
You can read the 2012 report on the Public Health England (PHE) website.
The Million Women Study, a national study of women's health involving more than 1 million women in the UK aged 50 or over, has currently found no association between the use of mobile phones for many years and the risk of brain tumours or any type of cancer.
The MTHR's set of volunteer studies of brain function is one of the largest carried out anywhere.
The studies found exposure to radio frequency fields generated by mobile phones had no detectable effect on brain function.
They looked at factors such as memory and response times, and found no changes.
The MTHR's research found no evidence that radiofrequency radiation from mobile phones or masts causes unpleasant symptoms.
Its research programme included some of the largest and most robust studies of this question.
The MTHR recognised specific concerns about TETRA radios and base stations used by emergency services, but the report released in 2014 said there's currently no evidence of specific adverse effects related to exposure to TETRA signals.
The Stewart Report noted that a small number of experiments suggested radio waves from mobile phones could cause biological effects in cells and animals.
The MTHR commissioned careful studies of 2 possible cellular effects identified in the Stewart Report: stress protein production and calcium signalling.
Stress proteins are produced when cells experience an increase in temperature.
Previous research suggested these proteins were produced in nematode worms when exposed to mobile phone emissions thought to be too weak to result in significant temperature rises.
But the studies supported by the MTHR showed the stress proteins were in fact produced as a result of a slight temperature rise (around 0.2C) caused by radio wave exposure.
Since this was already a well-documented effect and considered harmless, the MTHR did not propose further research in this area.
Calcium signals produced by mammalian cells are important in controlling various functions of the cells.
Research published in 2010 found no evidence that exposure to radio waves had any effect on these signals.
Levels of exposure to radio wave radiation from mobile phone masts (base stations) are generally much lower than from mobile phones and are well below international guidelines.
Audits of the amount of radiation produced by base stations in the UK found the radiation produced is generally less than 0.005% of the guideline values.
The MTHR reports stated that the biggest known threat mobile phones pose to health is from their use when driving, as using them at the wheel impairs driving performance and increases the risk of accidents.
There's no statistical evidence that mobiles are more of a distraction than a conversation with a passenger, but passengers are normally aware of traffic conditions and are therefore likely to stop talking in potentially dangerous situations.
No, and research is continuing. Mobile phones have only been widely used for about 20 to 30 years, so it's not possible to be so certain about the safety of long-term use.
More research on the effects of mobile phones on children is also needed, as they're known to be more sensitive than adults to many environmental agents, such as lead pollution and sunlight.
Government advice is to be on the safe side and limit mobile phone use by children.
Although the programme was jointly funded by the UK government and the mobile phone industry, its management was overseen by an independent committee of scientists, including a representative from the World Health Organization, and the funders had no influence over the selection, interpretation or reporting of studies.