Concerns that terrorists could attack Britain's nuclear power network have failed to be addressed by plans for new nuclear plants, says Charles Clarke.
Mr Clarke was replaced as home secretary by John Reid in May
The former home secretary says the government's energy review fails to answer key concerns about a new generation of nuclear power stations.
In a lecture about Labour's future, Mr Clarke also criticises Gordon Brown for backing calls for new nuclear weapons.
He says there is no coherent case for deciding to replace Trident arms now.
His speech in London follows a magazine article in which he said Labour had alienated many of its supporters.
He says changing the leadership will not solve the Labour Party's problems and instead there needs to be a debate about future policies.
He denies claims by Ed Balls, the economic secretary to the Treasury - and one of Gordon Brown's closest allies - that such calls mean "navel gazing".
And he stresses his contribution to the debate is not part of any leadership bid.
The government energy review earlier this year gave the green light to a new generation of nuclear power plants, but Mr Clarke says he remains sceptical.
The problem of how to dispose of nuclear waste safely and effectively has still not been properly sorted out, he says.
Mr Clarke argues that the costs of nuclear are very great and it is unlikely any private firm would meet them, leaving the state having to stump up the funds.
The safety of plants must be guaranteed, he says.
"The power of nuclear energy makes it a natural magnet for terrorist activity of a variety of types," he says.
"We need to be absolutely certain that we can protect ourselves completely against that threat, the cost of which by the way also has to be met by the state.
"So I am genuinely sceptical and I do not think that the Energy Review answers these concerns adequately.
"On the basis of the information I have so far seen, I am not convinced of the case for proceeding to a new generation of nuclear power stations in this country.
"I believe that investment in conservation and renewable energy offers a more reliable route to the energy sustainability which this country needs."
In his speech, Mr Clarke is equally scathing about the way debate on a possible replacement for the Trident nuclear weapons system has been handled.
Chancellor Mr Brown used a lecture at The Guildhall in London to signal his support for keeping Britain's "independent nuclear deterrent".
But Mr Clarke says there needs to be a full consideration of all the options.
"Our resource and strategic allocation should depend on the conclusions of that consideration and should not be pre-empted," he says.
"In short a convincing argument for taking the step which the chancellor announced at the Guildhall has yet to be presented."
The former home secretary will also argue that the case for green taxes is now "unanswerable".
Good progress has been made on taxing fuel and it needs to be taken further, he says.
And the case for extra taxes on aviation is now "difficult to refute", he argues.