Sitting in his third-floor office in the new Department for Productivity, Energy and Industry in London's Victoria Street (the Department of Trade and Industry until last weekend), the Government's Chief Scientist spoke at length of his concerns about climate change, nuclear power, and in particular what he terms the energy gap.

Sir David King means what will happen to Britain's fight against global warming as, over the next 15 years or so, 11 out of Britain's current 12 nuclear power stations, which do not emit carbon dioxide, the main greenhouse gas, come to the end of their working lives.

As they are shut down, the proportion of Britain's electricity produced by nuclear " and thus CO2-free " will drop from around a quarter now, to around four per cent by 2020.

Can renewable energy systems, such as wind, wave and solar power, currently providing three per cent of Britain's electricity, provide enough CO2- free power in the meantime to fill the gap and enable the UK to meet its demanding climate-change targets? For if they cannot, Sir David says, a new generation of atomic power stations may be necessary.

The quietly spoken South African-born scientist, professor of chemistry at the University of Cambridge, is a practised Whitehall hand and was scrupulously careful not to give any direct public endorsement of his own to any new nuclear energy programme.

This is a decision strictly for ministers. But the stress he laid on the energy gap gave an insight into the Government's private thinking.

'I've always said in the past that, as we move to 2020, the proportion of nuclear energy going onto the grid, if there is no nuclear new-build, is going to drop from roughly 27 per cent to roughly four per cent,' he said. 'By 2020 we'll be left with Sizewell B [the nuclear plant in Suffolk].

'So the question in my mind and in many other peoples' mind is going to be, whether the renewables targets and the energy- efficiency targets will be sufficient to meet CO2 reductions, in the face of a falling percentage from nuclear power.

He went on: 'That gap in energy is imminent, and that's why [nuclear] is a live issue, I'd be ducking it if I wasn't to say that. And it may be that the conclusion would be reached that we need another generation of nuclear-fission stations.'

However, one generation would probably be sufficient, he said, and then a much-expanded renewables sector, and possibly nuclear- fusion power, would be able to take on the burden of the UK's carbon- free future.

'In other words, if we look into the long-term future, and project forward the work on renewables, and also project forward the work on nuclear-fusion power stations, I would imagine we would in the long term not need to continue with nuclear-fission power.'

The other question was the public acceptability of any new nuclear build programme, he said. 'I don't see any Government being prepared to go down a route on an issue like this without taking the public with them, so I think that is a key issue.' He said he could understand fears over waste and accidents.

'I'm not a great fan of nuclear,' he said. 'I can understand the public concerns, I can understand all the concerns about radioactive waste and so on. But I also feel that the climate change issue is so important that we really have to examine the potential use of nuclear.'

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