Tony Blair has been accused of suggesting that closing local A&E departments would improve the NHS.
In a speech to health officials, he said the health service had to "face these challenges of change".
But opponents of hospital closures said Mr Blair had turned "all logic on its head".
The Prime Minister said difficult decisions would have to be made to provide more specialised care for people when they need it.
But he insisted this "one-off" opportunity to improve the health service would ensure "the best is yet to come" for the NHS.
There was scorn from opposing political parties with the Liberal Democrats accusing Mr Blair of "chasing headlines" while the Tories said providing the best care did not mean depriving people of local emergency services.
Geoff Martin, of campaign group Health Emergency, said: "Claiming that closing local A&E departments, trauma units and intensive care facilities will improve services turns all logic on its head.
"People are fighting these closures in their tens of thousands up and down the country because they know that closing local services and increasing journey times puts lives at risk."
Mr Blair defended the health service changes in a speech to more than 100 primary care trust chiefs.
Speaking in London, he said: "I genuinely believe the best is yet to come. More lives saved, treating patients better and making sure that the NHS is the pride and envy of the world."
"Quality healthcare could only come from significant change. I do not underestimate the difficulties of all this. I feel this is a one-off - if we fall down this time the consent for taxpayers funding the NHS will diminish."
