Seriously ill people risk a "dismal fate" because so few bystanders are willing to attempt cardiopulmonary resuscitation (CPR), a charity has warned.
Fewer than half of people would perform CPR if somebody was suffering a cardiac arrest, leaving victims with little chance of survival, the British Heart Foundation (BHF) said.
A cardiac arrest happens when the heart stops pumping blood around the body.
The signs are if somebody suddenly collapses, is not breathing normally and is unresponsive.
New research from the University of Warwick has found that bystanders who witness a cardiac arrest perform CPR in only four out of 10 cases.
This compares with rates of almost three quarters (73%) in Norway, where survival rates are up to three times as high.
Every minute without CPR or resuscitation using a defibrillator cuts a person's chance of survival by around 10%.
The public's lack of knowledge and fear of helping means precious time is lost waiting for an ambulance to arrive.
After 10 minutes without CPR or defibrillation, a person has a 2% chance of survival at best.
Surveys suggest that most (62%) of UK adults admit they are worried about what to do if somebody collapses in front of them.
Some 59% fear they will make things worse by trying to help - even though doing nothing is worse by far.
Simon Gillespie, chief executive of the British Heart Foundation, said: "When someone collapses after a cardiac arrest, every second counts.
"Knowing simple CPR skills is vital to ensure that every person has the best chance of survival."
More than 30,000 cardiac arrests occur out of hospital in the UK every year, with fewer than one in 10 people surviving.
The BHF says up to 5,000 lives could be saved every year if CPR rates matched Norway.
The charity is educating more than 35,000 schoolchildren in the UK on Friday on how to recognise a cardiac arrest and perform CPR.