An estimated 620,000 people in the UK have a faulty gene that puts them at risk of developing coronary heart disease or sudden death, and most are unaware, a charity has warned.
The British Heart Foundation said the figure was 100,000 more than had been thought and could be even higher.
It said there was now a better grasp of the prevalence of inherited conditions.
A child of someone with an inherited heart condition can have a 50% chance of inheriting it themselves.
The foundation warned that the overall figure for those with the faulty gene could be much higher because of as yet undiscovered faulty genes and under-diagnoses.
Each week in the UK, around 12 seemingly healthy people aged 35 or under are victims of sudden cardiac death with no explanation, largely due to undiagnosed heart conditions.