The Greens say a vote for Labour could be letting in "new nuclear power stations through the back door".
Most of the UK's nuclear power stations will be shut by 2023
The party says Labour has failed on climate change and plans to build new stations to provide power while producing fewer greenhouse gases.
But it says stations produce waste that remains a danger for generations.
The Lib Dems oppose any new stations. Both Labour and the Tories say concerns about cost and waste must be addressed before any decision is made.
Of the UK's 14 ageing nuclear power stations, all but one will have shut by 2023.
Green parliamentary candidate Darren Johnson told the BBC: "We are really, really concerned that a vote for Labour could be letting in new nuclear power stations through the back door.
"Because they have completely failed on climate change, they are now planning for desperate measures.
"While it (nuclear power) produces energy for 30 to 40 years, it produces nuclear waste for thousands and thousands of years to come."
The Greens unveiled their last campaign billboard outside Parliament on Wednesday, reading: "Nuclear: Tony Blair's post-election surprise".
Mr Blair has said his policy has not changed since the energy White Paper two years ago, which left nuclear power on the back burner.
On Wednesday he told BBC Radio 4's Today programme he could not rule out a new generation of civil nuclear power stations.
But he said: "Personally, I find it hard to see how you are going to get consent for that unless you deal with the issue of the public concern over waste and you deal with the issue of cost."
There have been reports that if re-elected, Mr Blair would raise the issue when the government responds to its climate change policy review in the summer.
The government says the UK will meet the Kyoto targets on climate change but says it has slipped behind its own tougher targets.
Nuclear power is almost "carbon neutral" - government figures suggest nuclear generation reduces national carbon emissions by between seven and 14%.
But campaigners say the risk of accidents, and the toxic waste it creates, mean it is not acceptable as an alternative to oil and coal.
The Lib Dems' environment spokesman Norman Baker has said relying on nuclear power to tackle climate change is "like jumping from the frying pan to the fire".
The Conservatives have previously said they are in favour in principle of a new generation of nuclear power stations.
Last week party leader Michael Howard said they did not have the facts and figures on waste disposal and cost to make a decision, but said there may be a strong case for a national review of nuclear stations.