A major investigation into whether mobile phones and other wireless technologies affect children's mental development is getting under way.
The study - funded by the government and industry - will track 2,500 11- and 12-year-olds from September.
It will look at their cognitive ability - thinking skills, memory and attention - and then repeat the tests in 2017.
The researchers say "very little" is known about what impact these technologies have on children.
The World Health Organization has said research in this area is of the "highest priority".
More than 160 secondary schools in the outer London area will receive invitations to enrol pupils into the study.
Much of the research into mobile phone use has focused on adults and in particular, the risk of brain cancer. No evidence of harm has been established to date.
Nonetheless, NHS advice is that children under the age of 16 should use mobile phones only for essential purposes and, where possible, with hands-free kits.
The theory has been put forward that children's brains may be more susceptible because they are still developing.
This research - led by Imperial College London - will put that idea to the test by asking children and their parents about their use of mobile phones and wireless devices, such as tablets, as well as analysing data from operators.
The 11-12 age group is particularly important as many children get mobiles at that age to coincide with starting secondary school. Some 70% of that age group now own a mobile.
Lead investigator Dr Mireille Toledano said: "The advice to parents is based on the precautionary principle given in absence of available evidence and not because we have evidence of any harmful effects.
"As mobile phones are a new and widespread technology central to our lives, carrying out the study is important in order to provide the evidence base with which to inform policy and through which parents and their children can make informed life choices."
The researchers will now be writing to 160 secondary schools in the outer London area asking pupils to take part.