Many people with deadly inherited heart conditions are only diagnosed after having a cardiac arrest, research has shown.

One in five of those with such a condition only finds out after the death of someone in their family, the British Heart Foundation (BHF) said.

The charity estimates that around 620,000 people in the UK have a faulty gene which puts them at a particularly high risk of developing heart disease or dying suddenly at a young age.

Each child of someone with an inherited heart condition has a 50% chance of inheriting the same faulty gene – but the majority of people remain undiagnosed.

It is also estimated that at least 12 young people (those under 35) die from an undiagnosed heart condition every week in the UK.

The figures have been released as part of the charity’s In Your Genes campaign, which aims to raise awareness of inherited heart conditions and highlight the urgent need for better diagnosis.

BHF is urging people to speak to their family about any early sudden deaths or premature heart disease in the family, which might have been caused by an inherited heart condition.

Once someone has been diagnosed with an inherited heart condition, genetic testing should be carried out on all first-degree relatives – a process called cascade testing.

BHF medical director Professor Sir Nilesh Samani said: “It is extremely important that family members are offered genetic testing when there’s a history of sudden death or premature heart disease in a family.

“Once diagnosed, many of these conditions are manageable with medication or ICDs (implantable cardioverter defibrillators), and can prevent needless deaths.

“All too often, people aren’t familiar with their family history, or they aren’t aware that a sudden death might be linked to an underlying heart condition.

“We need to improve awareness of these conditions, and ensure that people have equal access to cascade genetic testing across the UK.”

Owen Steatham, from Birmingham, was just nine when he collapsed after suffering a cardiac arrest on his way home from school.

Luckily, paramedics were at the scene quickly and managed to save his life using a defibrillator.

Tests revealed that Owen, now 13, was living with the inherited heart condition hypertrophic cardiomyopathy (HCM) and he has now been fitted with an ICD.

As HCM is an inherited heart condition, the rest of Owen’s family were tested and it was revealed that his father and younger brother, Riley, seven, are also carrying the faulty gene. They are now being monitored so they can be treated if they too develop the condition.

Owen’s father, David, 37, said: “We know that genetic testing has saved lives in our family. My uncle was diagnosed with the condition after Owen’s diagnosis, which led to him having an ICD fitted.

“Since then his ICD has delivered a shock to his heart after it went into a dangerous rhythm. Without the diagnosis and ICD he might not be alive today. Owen has also had 10 shocks from his ICD since his diagnosis, which has literally saved his life.

“I am so incredibly grateful to the genetic testing service. No-one should have to go through what Owen went through.”

Anyone with a history of unexplained sudden deaths in their family should speak to their GP, or call the BHF Genetic Information Service on 0300 456 8383.