Tony Blair is believed to be convinced over the need for nuclear power to tackle the UK energy crisis.
The debate over nuclear fuel has gone on for too long, the CBI says
The government is to announce a review of energy policy, including nuclear power, after being urged by business leaders to tackle the UK energy crisis.
Concerns have been growing over future power supplies and rising gas costs.
The BBC's Nick Robinson said despite the prime minister's support, no decision has yet been made on Britain's nuclear future.
Tony Blair's spokesman said: "The prime minister's view is that we need to look at all the options and everybody knows that is what we are going to do."
He said it was important to look at it in terms of the UK's energy security and also "in terms of climate change".
Government Chief Scientist Sir David King told the BBC that a "fresh look" was needed at the situation but denied that any firm decisions had been made ahead of the review.
"My advice has been clear for some time, but I don't believe that decisions have been made, " he said.
Earlier he had urged the government to "give the green light" to more power stations.
The CBI, the business lobby organisation, says energy requirements are now top of the business agenda as fuel costs rise and worries grow over gas supplies this winter.
Mr Blair is expected to use the CBI annual conference next week to announce the energy review and signal the government's change of direction.
BBC political editor Nick Robinson says that the Prime Minister has been convinced that building more nuclear power stations is the only way to meet the country's energy needs and stick to the targets on climate change.
The CBI has stressed a firm decision on a new generation of nuclear stations must be made urgently.
It said one-third of the UK's generating capacity would have to be replaced by 2020 and called on the government to commission a study into the cost of nuclear energy compared with alternative sources of power.
"A decision on the future of nuclear power has been allowed to drift too long," said the CBI's director general Sir Digby Jones.
"Potential investors and the British public both deserve certainty."
He told BBC Radio Five Live: "It is high time this nation had an integrated coherent energy policy."
And he warned that high-use large industrial outfits would have to "throw the switch" if the price of gas continued to rise.
"This government is going to have to hold a proper constructive debate on nuclear power. We want them to have a public debate and stop prevaricating."
The call comes as the price of wholesale gas has almost doubled during the past week, prompting fears about winter supplies to industry in the UK.
Experts believe tight supplies have triggered the rise.
UK supplies are low as a pipeline from Europe is running at half capacity and shiploads of gas are being diverted to Spain and the US where prices are high.
UK energy minister Malcolm Wicks said the government was looking into why the gas interconnector was not working properly, but said it was operated by private companies and "is not something the government switches on and off".
He admitted that the rundown in North Sea supplies and the delay in getting new pipelines from Norway up and running meant that some sectors of UK industry may experience a difficult winter or two.
"We have got a tight equation between supply and demand of gas," he said.
Former Labour energy minister, Brian Wilson, told the Midday News on Radio Five Live he hoped the government would "give a clear steer in favour of nuclear power stations".
He added: "Both in order to meet our environmental responsibilities but also to maintain security of supply and avoid this gross over-dependence on gas."
But former environment secretary Michael Meacher said that while the government had "to act quickly... I think we need nuclear like a hole in the head".