CT scans can triple the risk of children developing leukaemia and brain cancer according to a Manchester University professor.
There is a very small risk of developing health problems from the radiation in CT scans but scientists have urged for better protection.
It was presumed that low-dose CT scans caused an insignificant risk to children, who are more sensitive to radiation than adults.
Professor Richard Wakeford, from the Dalton Nuclear Institute at the University of Manchester, said: "The measures taken to protect against radiation exposure assume that any dose of radiation, including from exposures for medical purposes, carries with it some risk, but that the risk from low doses is small.
"This means that large studies of many people are needed to detect this presumed radiation risk against the background risk from other factors, and this is what this large CT scan study has achieved.
"The radiation risks the study has detected are at the level currently assumed for the purposes of radiation protection, i.e. small, which is reassuring. However, as with all sources of radiation exposure, this small risk should be taken into account when radiation is used."
The team of scientists – headed by Britain – studied data on nearly 180,000 patients under the age of 22 who had CT scans at UK hospitals between 1985 and 2002.
The rates of cancer that developed were compared with those from the general population, reported in the UK National Health Service Registry.
The findings revealed children younger than 15 receive enough radiation from two to three head CT scans to triple their danger of developing brain cancer.
The research did not analyse why children underwent CT scans, but insisted they already had brain tumours or leukaemia.
Lead researcher Mark Pearce, from the University of Newcastle, said: "Further refinements to allow reduction in CT doses should be a priority, not only for the radiology community, but also for manufacturers. Alternative diagnostic procedures that do not involve ionising radiation exposure, such as ultrasound and MRI (magnetic resonance imaging) might be appropriate in some clinical settings.
"Of utmost importance is that where CT is used, it is only used where fully justified from a clinical perspective."
CT scans were have been greatly used over the past decade after being introduced in the 1970s.
The number of UK scans rose by 68% between 1998 and 2008 and in 2007 approximately 72 million scans were carried out in the US.