There is a deafening, unearthly howl as if a jumbo jet was firing up its engines in London's Albert Hall. On the screen in the control room a ghostly pinkish glow whips round the edges of the inside of the nuclear reactor. At its core it is 10 times hotter than the centre of the sun.

This, according to some physicists, is the solution to the energy crisis a future with cheap, reliable, safe and nearly waste-free power. Today, after years of false starts and political wrangling dating from the Cold War, they will get their chance to make that dream a reality.

A US$14 billion project, called Iter, to build a prototype nuclear fusion reactor will be signed off in Brussels by the EU, Japan, China, South Korea, India and the United States.

The prospect of virtually limitless energy is not merely science fiction. The haunting, screaming growl of matter being smashed together at unimaginably high speed is a daily occurrence at Jet in Oxfordshire, Southern England, an existing experimental fusion reactor. Jet is by far the biggest of the world's 28 fusion reactors. It is the work of scientists here that has paved the way for the much bigger Iter, which, once the project is ratified in December, will be built in Cadarache in southern France.

Its advocates say nuclear fusion is the most promising long-term solution to the energy crisis, offering the possibility of abundant power from cheap fuel with no greenhouse gases and low levels of radioactive waste. But critics say the government is gambling huge sums of money 44% of the UK's research and development budget for energy on a long shot with no guarantee of ever producing useful energy.

Last week British Prime Minister Tony Blair backed conventional nuclear power, saying in a speech to business leaders that not replacing Britain's ageing nuclear power stations would be "a serious dereliction of our duty to the future of this country."

He argued that only nuclear energy could prevent a huge hike in CO2 emissions once the current nuclear stations were decommissioned.

But while the debate over the future of conventional nuclear power continues, many physicists argue that fusion is the future. "Fusion works it powers the sun and stars," said Sir Chris Llewellyn Smith, head of the UK Atomic Energy Authority. "In the second part of the century I'm optimistic it will indeed be a major part of the world energy portfolio."

Unlike nuclear fission, which tears atomic nuclei apart to release energy, fusion involves squeezing the nuclei of two hydrogen atoms together. This process releases a helium nucleus and a neutron plus huge quantities of energy.

The hydrogen fuel is part heavy hydrogen or deuterium, which can be easily extracted from water, and part super-heavy hydrogen or tritium, which can be made from lithium, a reasonably abundant metal.

The energy produced is truly colossal. The lithium in just one laptop battery and the heavy hydrogen from half a bath of water could provide enough energy for the average European for 30 years.

One of fusion's big advantages over fission is safety. Firstly, there is no chance of a runaway meltdown as happened at Chernobyl.

If you stop applying the fuel or switch off the magnetic jacket that keeps the fuel in the reactor, the reaction just stops.

"It is very difficult to keep it running. It is like keeping honey on the back of a spoon," said Mathias Brix, a physicist at Jet. Also, the quantities of fuel involved are much smaller than in fission reactors. Jet contains less than a gram of fuel, while Chernobyl had 250 tons. Lastly, the fuel and waste from the reactor is much less radioactive.

But although physicists think they understand fusion, harnessing it has proved extremely difficult. Research first began in the 1950s with claims that fusion would provide reliable power by the end of the century but even now scientists admit that a commercial application is at least 40 years away.

The problem is getting two nuclei close enough to fuse and then controlling the reaction. This means putting in huge amounts of energy at the start to convert less than a gram of the fusion fuel into a super-hot gas or plasma. Hydrogen nuclei flying around at high speed in the plasma can then come close enough together to fuse.