Ireland's Data Protection Commissioner, Helen Dixon, is examining a report that Yahoo Inc scanned emails for US intelligence officials and said it would view any infringement of European Union citizens' privacy rights as a "matter of considerable concern".

Reuters reported on Tuesday that Yahoo secretly built a custom software programme to search all of its customers’ incoming emails for specific information provided by US intelligence officials, according to people familiar with the matter.

“Arising from media reports, the Office of the Data Protection Commissioner is making inquiries into this matter,” the lead European regulator on privacy issues for Yahoo – whose European headquarters are in Dublin – said in a statement.

“Any form of mass surveillance infringing on the fundamental privacy rights of EU citizens would be viewed as a matter of considerable concern by this Office.” Yahoo said in response to the original Reuters story that it is a law-abiding company that complies with US laws.