People with pacemakers should hold their mobile phones as far from their chests as possible, Queen Mary Hospital researchers warned.
Studies on 29 heart patients showed mobile phones had caused pacemakers to beat faster or slower - and, in one case, to shut down for several seconds.
Queen Mary cardiologist Dr David Ho Sai-wah said the risk of a dangerous stoppage could be reduced by turning the head and holding the mobile phone on the opposite side of the body.
'During our study, when the mobile phone was placed literally next to the implanted pacemaker, it stopped for a few seconds on one patient. Such a pause can make a person faint,' Dr Ho said.
But if patients kept their phones on the opposite side to their pacemakers 'the effect will then be too minimal to worry about', he said.
'Of course, the safest way is not to use mobile phones at all.'