The UK will experience prolonged power cuts in about five years unless urgent action is taken now, a report warns.
It said a third of generation capacity was due to be decommissioned by 2020, but was not being replaced fast enough.
The report, by nuclear supporting Fells Associates, said new reactors would not be ready in time, and questioned spending on renewable energy.
Greenpeace UK said Britain had been “left behind” on climate change, and must be a leader on renewable energy.
The report was commissioned by Sheffield-based industrialist Andrew Cook, who voiced concern about a “fearful void” in energy policy.
The report – A Pragmatic Energy Policy for the UK – was compiled by Fells Associates, a network of energy and regulatory specialists.
Co-author Candida Whitmill said the so-called “energy gap” would also have severe economic consequences.
“The current credit crunch is a head cold compared to the double pneumonia this country will suffer if we don’t implement an energy policy urgently,” she told reporters.
“That is why security of supply now takes priority over everything, even climate change.
“If we are going to cope with climate change, it is going to cost money; if we want to protect the environment, it is going to cost money; and if we want to change to a low-carbon economy, it is going to cost money.”
The report identified a number of factors that would combine to create the energy gap.
It said the main impact would be the loss of 23 gigawatts (GW) of electricity generation capacity between now and 2020.
The UK’s ageing nuclear reactors, which currently provide about a fifth of the nation’s electricity, are set to be decommissioned over the coming years.
Current projections show that by 2023, the UK will have only one nuclear reactor in operation.
And an EU Directive that requires the most polluting coal- and oil-fired power station to close would result in the likely loss of a further 12GW generation capacity.