Radical reform of the NHS is needed because England has fallen behind other European countries in its healthcare provision, the Prime Minister said.
Defending plans to pass control of commissioning care to GPs, David Cameron said the medical profession has shown "enthusiasm" towards the changes, which will be brought in across England in 2013.
He said 141 GP groups have asked to take on new commissioning powers ahead of the 2013 date, and claimed this shows they are keen to embrace decision-making on the front line.
Mr Cameron rejected claims from the Royal College of GPs, the British Medical Association and trade unions that the shake-up is unnecessary, saying: "I think if we just carried on as we are... we would face a really big crunch in two or three years' time.
"It is necessary because we have fallen behind the rest of Europe. We have spent similar amounts of money, we are more likely to die of cancer or heart disease."
Responding to criticism about the speed of reform, he said the previous government had waited too long to introduce changes and said the measures are being introduced "steadily".
He said the changes would save money within two years by reducing bureaucracy.