Nearly two thirds of Britons don't know what to do if someone's heart stops beating, according to a first aid charity.
St John Ambulance also says more than half of people don't know where their nearest defibrillator is.
Charity director James Radford said: "What we're trying to do is to encourage the very simple steps that everyone could take: find out where your closest defibrillator is, know how to spot the signs of a cardiac arrest, be ready to use CPR to resuscitate, so know that early defibrillation can make all the difference in the terms of survival."
Awareness of the problem has grown since the sudden collapse of former footballer Fabrice Muamba in 2012.
His heart stopped for 78 minutes and was saved thanks to the immediate care given to him on the pitch and the use of a defibrillator 15 times.
But knowledge in Britain is still behind that of other countries, Mr Radford said.
"In this country, fewer than one in 10 people survive when they have an out of hospital cardiac arrest.
"That doesn't compare very well internationally but we do know why it could be improved and how it could be improved.
"This report highlights just how unprepared many in the public actually are if they were faced themselves with someone having a cardiac arrest."
Judith Mansfield knows exactly where her nearest defibrillators are.
The 72-year-old from Burnham-on-Sea had a sudden cardiac arrest whilst at the theatre with her daughter in January 2016.
Her daughter Beverley said: "I think mum was just so lucky – if you want to call it that – that she was there with somebody, there was a nurse in the audience who recognised the signs, St John Ambulance volunteering, defibrillator on site, and the paramedics and the heart paramedics especially were there very, very quickly.
"I think if it had happened anywhere else the outcome would be totally different."
Judith wants others to be prepared.
She said: "I do feel lucky to be here.
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"I wake up in the morning and think 'another day', I really feel lucky.
"I'm not ready to go yet."