The private sector could not shoulder the full clean-up costs of new nuclear power plants in Britain, potential investors say, casting doubts on government claims last week that it would not subsidise new reactors.

The big problem is no one knows what the full decommissioning costs will be - Britain is still consulting on both near-term and permanent waste disposal options at current sites, and research has not begun into such costs at new sites, according to engineering firm AMEC PLC.

"The government needs to tell us what to do with the waste, and that will affect the sums. If the sums don't add up we won't do it," said Jonathan Smith, spokesman at E.ON UK, whose German utility parent E.ON owns nuclear plants in Germany and is a potential investor in Britain's nuclear sector.

All but one of Britain's aging reactors, which produce a fifth of the UK's electricity, are due to shut by the 2020s. Prime Minister Tony Blair, under pressure to deliver a strategy that will both keep the lights on and cut greenhouse gas emissions, has signalled he will back the construction of a new fleet of reactors in a policy review due out next month.

But Energy Minister Malcolm Wicks ruled out a direct subsidy for new reactors - including decommissioning costs when a plant retires - while the Department of Trade and Industry would not comment on whether that excluded any public backing.

Likely, private-sector players say they would support only limited contributions to uncertain decommissioning costs for new plants, possibly through a levy during the 40- to 60-year lifetime of plants.

"It would not be unpalatable to have a tariff mechanism where at the end of the plant lifetime the tariff takes care of what it can and the NDA (Nuclear Decommissioning Authority) takes over the liability," said Gerry McGill, managing director at the nuclear division of AMEC.

The UK's biggest power producer, British Energy, another possible new build backer, owns eight nuclear plants and contributes 65 per cent of its net cash flow to a decommissioning fund - in a possible model going forward.

AMEC was involved in the design and construction of Britain's existing nuclear power plants and wants a big role in any new build, but says the private sector needs a limited liability guarantee.

"You'd want a reasonably robust contract," said Jim Wright, AMEC strategy and development director.