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.
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."
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.