The UK would need almost 50,000 more nurses and 26,500 more doctors to match standards in the rest of the developed world, according to new figures.
Experts said the quality of care in the UK is "poor to mediocre" across several key health areas, while obesity levels are "dire" and the NHS struggles to get even the "basics" right.
A new report from the Organisation for Economic Co-operation and Development (OECD) compared the quality of healthcare across 34 countries.
It found the UK lags behind in key areas, including coming 21st out of 23 countries on cervical cancer survival, 20th out of 23 countries on breast and bowel cancer survival and 19th out of 31 countries on stroke.
On heart attack deaths, the UK is ranked 20th out of 32 countries. While survival after hospital admission for heart attack and stroke is improving, it is "worse than many other OECD countries", including Canada, Italy, the Netherlands and Spain, the study said.
The UK is also a "middling to low performer" on three out of four key health areas, including quality of care and life expectancy, and there is a need for "urgent attention" to tackle the UK's high rates of smoking, harmful alcohol consumption and obesity.
Some 19% of adults in the OECD are obese on average, but the figure in the UK is 25%.
Spending on health is also lower in the UK than the OECD average, with "zero growth in health spending per person in real terms between 2009 and 2013", the report said.
Countries that spend more include France, Canada, Belgium, Germany, New Zealand and Denmark.
The UK also has a lower than average number of doctors and nurses.
Nigel Edwards, chief executive of the think tank the Nuffield Trust, who helped launch the report at a meeting in central London, said thousands more doctors and nurses were needed to even match the OECD average on staffing.
The UK would need 47,700 more nurses and 26,500 more doctors to match the OECD average, he said.
This would cost an extra £5 billion a year.
The OECD average number of nurses is 9.1 per 1,000 population, while the figure in the UK is 8.2.
The UK also lags behind on numbers of doctors, with 2.8 per 1,000 population, compared to the OECD average of 3.3.
Countries with more doctors include Greece, Italy, Germany, Spain, Portugal, Switzerland, the Czech Republic, Israel and Australia.
Mark Pearson, OECD deputy director of employment, labour and social affairs, said the UK was doing "outstandingly poorly" on preventing ill health by tackling issues like obesity.
He added: "While access to care in the UK is good, the quality of care is uneven and continues to lag behind that in many other OECD countries."
He said the UK "often does not do the basic things very well", and said the lower than average levels of public investment in healthcare was mirrored by a "somewhat mediocre performance across the board - from relatively low staffing levels, to high rates of avoidable admissions for asthma and lung disease".
Mr Pearson said the UK "can and must do much better", adding that in healthcare "you get what you pay for".
The UK was spending "considerably less" on health than many of the countries it would like to compare itself to, he added.
Mr Pearson pointed to NHS achievements, including being efficient in some areas, such as lengths of stay in hospital and good performance on avoidable admissions for diabetes.
But while screening rates for NHS cancer screening programmes were high, survival rates from cancer were below average.
Overall, staffing levels were not adequate and there were still too high rates of hospital-acquired infections. Guidelines were also published but not followed, he said.
"These basic sort of things don't seem to be done to the extent in the UK that they are done in other countries," he said.
Mr Pearson said lower levels of spending and lower levels of staffing in the UK meant health workers did not have time to compare their performance with other local hospitals or those on an international level.
He added: "I think it's unrealistic, if you spend less, that you can expect a performance that is as good as these other countries."
Mr Edwards added: "If people are stretched, people don't have the bandwidth to make a change."
He said calculations showed that spending on health was going to remain flat in real terms over the next five years.
And he added that junior doctors were relied upon to provide healthcare.
"What's unusual about British hospitals is that when you see someone, they are quite junior.
"We use juniors to provide the backbone of the workforce in hospitals rather than (more highly qualified) doctors."