The Government was reported to be worried that without a “carbon levy” on conventionally-produced power, nuclear firms could be reluctant to invest in the UK.

The Office of Nuclear Development (OND), set up by Lord Mandelson’s business department, had told firms it would keep the price of carbon in the EU emissions trading above 30 euros per tonne.

Energy consultancy firm EIC calculated the carbon levy would add £44 to the £500 annual electricity bill paid by an average household.

It suggested the need was now considered still more urgent amid growing doubts that the United Nations-sponsored climate change summit in Copenhagen in December would reach an agreement on carbon.

A “senior government source” said: “We know what companies’ investment criteria is. If Copenhagen does not reach agreement on it we will do it next year, either via a subsidy, levy or tax. We will look at it - the utilities know it.”

A spokeswoman for the Department of Energy and Climate Change said it there were “no current plans of this sort”.

The UK was “going all out for an ambitious deal at Copenhagen”, she added.

Torness Nuclear Power Station, East Lothian