By Alex Kirby BBC News Online environment correspondent

A decision is close on the future of the UK's energy supplies for the next half century.

A report the government is expected to publish on Thursday advocates a big expansion of renewables like wind and solar power.

But it also suggests that nuclear energy could help Britain to fulfil its international commitments.

The government is already split on which option to follow.

The report, a review of future energy prospects, was prepared by the Cabinet Office's Performance and Innovation Unit (PIU).

It considered the implications of a report by the Royal Commission on Environmental Pollution, which urged a 60% cut over the next 50 years in the UK's emissions of carbon dioxide (CO2).

Many scientists believe CO2 is the chief human contributor to global warming.

Ambitious target

Under the international agreement on tackling global warming, the Kyoto Protocol, the UK must reduce emissions of six greenhouse gases to 12.5% below their 1990 levels over the next decade.

The government has given a separate pledge to cut the country's CO2 emissions by 20% by 2012.

Wind energy will be cheaper than nuclear

A final draft of the PIU review, seen by BBC News Online last December, said: "Credible scenarios for 2050 can deliver a 60% cut in CO2 emissions, but large changes would be needed both in the energy system and in society."

On renewable energy the existing target is for 10% of electricity to come from renewable sources by 2010.

The draft proposed that by 2020 renewables should be supplying about 20% of the UK's electricity, adding perhaps about 5-6% to domestic electricity bills.

It included a section entitled "Nuclear power - keeping the option open".

Opposite corners

Some commentators believe the qualifications the PIU added (public acceptance and commercial willingness to accept the risks) meant the option was effectively closed.

Energy Minister Brian Wilson, who supports the nuclear industry, argued its case as the final draft was amended before going to Mr Blair. Environment Minister Michael Meacher has spoken strongly for renewables.

The review spells out the dilemma: the UK will find it difficult to meet its international promises, but neither public opinion nor the market is ready to back enlargement of the existing fleet of power stations, most of them needing replacement by about 2008.

Solar power has huge potential

Stewart Boyle, an energy analyst and writer with Platts Energy, told BBC News Online: "The review recognises that many people continue to be worried by the existing stockpile of nuclear waste.

"It implies that there is no hope of winning public acceptance for a new generation of reactors until people are confident the waste problem can be solved.

"Then there's the question of what sort of reactor could replace the present ones, the pressurised water reactors in use at Sizewell B, the advanced gas-cooled reactors, and the old Magnox stations.

Reassuring investors

"They're all too expensive for the liberalised energy markets. British Energy and BNFL would prefer a newer type pf pressurised reactor, the AP600, made by Westinghouse.

"There are signs that Westinghouse might seek to build a demonstration AP600 somewhere to show the UK that it can work safely and economically.

"But that could take until the end of this decade, so nuclear isn't a short-term carbon solution in the UK."

The Cabinet is expected to announce its energy policy later this year or early in 2003.