The Tories appear to be weakening their traditional support for nuclear power as debate on a new plants gathers pace.
Mr Blair has raised the prospect of new nuclear power stations
Shadow trade and industry secretary Alan Duncan says he opposes subsidies or price guarantees for nuclear firms.
His words came as Tony Blair announced a new agreement with France to share nuclear expertise.
Speaking after talks with French President Jacques Chirac, Mr Blair said it showed energy policy was "right at the top of the agenda".
Mr Blair denied he was prejudicing the government's energy review, which is due to report next month.
But he told reporters in Paris he would face "a very big problem" if he was expected to rule out the replacement of Britain's ageing reactors.
"But I think that this is a classic case that the decisions we take today as political leaders will be felt in 15, 20 or 30 years' time and I don't want people looking back and saying 'what were those guys doing, when the facts were very clear and very obvious to them?'."
'Honest economics needed'
Earlier, Mr Duncan said he did not think a cross-party consensus was needed before new nuclear plants could be built.
But he made clear the Tories did not think governments should underwrite the nuclear industry.
"We, like the government, are not looking at a subsidy regime for nuclear power," he said.
"They must justify their own arguments with honest economics and a proper regime for their waste.
"That means no price guarantee and no volume guarantee for nuclear power. Everyone to do with nuclear power has to answer difficult questions about the honesty of the economics and the safety of their waste handling."
But a spokesman for the Nuclear Industry Association said it was not asking for subsidies or price guarantees.
It was confident a new generation of plants could be built without financial help from the government, he said.
Mr Duncan's comments appear to make cross-party consensus on building a new generation of nuclear power plants unlikely.
The Liberal Democrats have already voiced their opposition to new stations.
Leader Sir Menzies Campbell said this week: "New nuclear power is not the answer to climate change. It would come too late. It is far too expensive. And it is just too risky."