A fifth of people witness somebody collapse and need CPR but most of them do absolutely nothing to help, according to worrying new research.
An estimated one in five adults in the UK witness someone collapse who needs immediate CPR, yet the majority of people do not act, according to the British Heart Foundation.
The worry surprising findings have been released today (Monday) on Restart a Heart Day - an annual day to increase awareness of the importance of CPR.
This year, more than 150,000 young people across the UK will be trained in CPR in the largest ever event of its kind.
Cardiac arrest survival rates in the UK have remained low and a collaboration of leading organisations are calling for all young people to be trained in CPR to help save more lives.
Prof Gavin Perkins, Professor of Critical Care Medicine at the University of Warwick, who led the research, said: "Our research shows just how important it is for everyone to learn CPR. It is staggering to think that one in five of us will at some point have the opportunity to save a life by giving CPR.
"CPR is a vital step in the chain of survival after a cardiac arrest.
"The chance of surviving is almost zero if people collapse and receive no bystander CPR until the emergency services arrive.
"Thousands of deaths could be prevented if more people learn CPR."
Researchers carried out a survey of 2,000 people across the country to find out how likely people are to witness a life-threatening cardiac arrest.
In addition to the vast numbers of people who have seen someone suffer a cardiac arrest, they also found that people were nearly three times more likely to perform CPR if they had relevant training - boosting potential survival rates from one in ten.
They estimate that 10,000 people die every year in the UK as rates of bystander CPR are as low as 39 per cent in some parts of the country, which is lower than many other Western countries.
Simon Gillespie, Chief Executive at the British Heart Foundation, said: "CPR is the difference between life and death for thousands of people every year in the UK who suffer a cardiac arrest.
"Every second counts, and it simply isn't enough to hope that someone who knows CPR is present.
"We need everyone to learn this life-saving skill to give them the confidence to step in and give CPR when someone collapses after a cardiac arrest.
"That's why we are urging secondary schools across the UK to apply for our free training kits and help create a Nation of Lifesavers."