Around 620,000 people in the UK have a faulty gene that puts them at risk of sudden death, a charity has warned.
Most people are unaware they carry the faulty gene - which puts them at high risk of heart disease, heart attack or cardiac arrest.
The British Heart Foundation (BHF), which released the figure, said it was 100,000 more than previous estimates. But it said the true figure could be even higher due to under-diagnosis and the presence of other, as yet unknown, faulty genes.
Last April, former England and Nottinghamshire batsman James Taylor was forced to retire due to a rare but serious genetic heart condition.
Scans revealed he has ARVC (arrhythmogenic right ventricular arrhythmia), an inherited condition caused by a change or mutation in one or more genes.
It means the right side of the heart fails to pump blood around the body properly and can cause abnormal heart rhythms.
The condition is progressive, which means it will get worse over time, and there is a risk of sudden death.
In its new warning, the BHF said inherited heart conditions can affect people of any age. Children have a 50% chance of inheriting the faulty gene from a parent who has it.
Around 12 people aged 35 or under die every week with no apparent explanation, mostly due to these inherited genetic faults.
Professor Sir Nilesh Samani, medical director at the British Heart Foundation, said: "The reality is that there are hundreds of thousands of people across the UK who are unaware that they could be at risk of sudden death.
"If undetected and untreated, inherited heart conditions, can be deadly and they continue to devastate families, often by taking away loved ones without warning.
"Thanks to the public's kind support, BHF-funded researchers have discovered some of the genes responsible for these frightening conditions but there is still much to do.
"We urgently need to fund more research to better understand these heart conditions, make more discoveries, develop new treatments and save more lives."
Taylor 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."
The BHF does not recommend nationwide screening for genetic conditions but says close family members of anyone found to have a fault should be tested.
A spokesman said: "If someone in your family has been diagnosed with or has died from what is suspected to be an inherited heart condition, you should speak to your GP or call our genetic information service for more information and support."