Around 620,000 people in the UK have a faulty gene that puts them at risk of sudden death, warns a leading heart charity.
The number is 100,000 more than previous estimates, said the British Heart Foundation (BHF), but the true figure could be even higher due to under-diagnosis and the presence of other, as yet unknown faulty genes.
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"If undetected and untreated, inherited heart conditions can be deadly and they continue to devastate families, often by taking away loved ones without warning."
He added that without more "urgently needed funding", a greater number of heart defects will go undiscovered.
Last year, England test cricket batsman James Taylor was forced to retire due to a genetic heart defect.
Scans revealed he has arrhythmogenic right ventricular arrhythmia (ARVC}, an inherited condition caused by a change or mutation in one or more genes.
He said: "It is safe to say that being diagnosed with ARVC was the toughest and scariest week of my life.
"I never would have thought it would happen to me. I was 26 years old and playing cricket for England but my condition meant that I was at risk of sudden death from a cardiac arrest.
"I was lucky as my condition was detected early and, despite having to give up my career, with medication I can lead a relatively normal life. But it could have been an incredibly different story."
BHF-funded research has helped pioneer technology such as implants that treat a dangerous heart rhythm, as well as discovering many of the faulty genes that cause inherited heart conditions, says ITV News.
It has also launched a new television campaign to raise awareness of the devastation caused by heart disease.