Britain could end up with an “apartheid” pension system unless reforms are pushed through in the teeth of trade union opposition, David Cameron said.
The Prime Minister said public sector workers had to show the same "ethic" as private sector employees in working longer.
Union leaders are discussing further walkouts because of continued opposition to the Government's reforms after November's mass strike.
Mr Cameron spoke out during a discussion about managing the ageing population at the Northern Future Forum in Sweden.
"We do have one problem with the public sector pensions system where you have got a lot of resistance to changing public sector pensions, some of which have very low retirement ages," he sad.
'We think we are making some progress, otherwise we could end up with quite an apartheid system where people in the private sector have this flexible ethic, they go on working, they change the way they work. But in the public sector, we have quite a cut-off and a very expensive public sector pensions system."
Mr Cameron also expressed a keen interest in a Norwegian pension reform which automatically links provision to the rise in life expectancy.
"I love the idea," he told the panel, which also included Norwegian prime minister Jens Stoltenberg.
Norway has a flexible state pension system which allows people to choose the age at which they start to claim it - with higher payments to those who choose to wait the longest, up to 75.
The level of payments is automatically adjusted according to changes in life expectancy.
A spokesman for the Public and Commercial Services union said: "It's unfortunate that our Prime Minister feels it's acceptable to use the language of political oppression to describe the livelihoods of millions of public servants.
"Private sector workers have not chosen in an ethical way to have their pensions slashed - it's been forced on them by profit-hungry executives and shareholders. We want fair pensions for everyone."
Mr Cameron said he was especially interested in the Norwegian pension system because it "created a process rather than a cliff edge".
Pressed at the post-summit press conference, he said: "I don't think anyone is saying 'You must work until you're 75'. I think what we are all saying is that we need flexibility, we need to have a system that allows people greater choices, greater chances.
"Many people in the UK choose to work past their retirement not least because they are able to take their pension and work.
"No-one is talking about people who do hard manual labour continuing in those jobs year after year but we should give people better chances and choices to retrain and do different things."
That included people "of huge experience and talent" leaving the Armed Forces, he said.
"It would be a sad outcome if all the headlines... were 'Leaders say work until you drop'. That is not what we are saying."
A visibly embarrassed Mr Cameron at that point felt the need to apologise to his fellow leaders as question after question was aimed directly at him by media from all the nine countries.
"Sorry for hogging the questions," he said as he was grilled on everything from Syria and the Falkland Islands, to the resignation of England football manager Fabio Capello and his taste in music.
"I am a fan of The Smiths; The Smiths aren't necessarily a fan of me," he told one reporter.