However it was still a lower proportion than in some other countries. In Japan, which has the highest annual exposure to X-rays in the world, the figure was put at more than 3%.

Diagnostic X-rays include mammograms and CT (computed tomography) scans.

While doctors generally accept there is a small risk of contracting cancer from an X-ray, the extent of the danger has been unclear.

Today's research in the Lancet medical journal is the most detailed assessment so far.

Dr Amy Berrington de Gonzales, from Cancer Research UK, and Professor Sarah Darby, from Oxford University, based their findings on the annual number of diagnostic X-rays taken in the UK and 14 other developed countries.

In an accompanying article, Peter Herzog and Christina Rieger from Ludwig-Maximilians University in Munich, Germany, said: "A general goal must be to avoid unnecessary X-ray procedures. Up to 30% of chest X-rays may not be indicated; unnecessary CT examinations can lengthen hospital stay as well as causing radiation exposure.

"In everyday practice, those ordering radiological procedures should think carefully about the benefit for and the risk to their patients for each examination."

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