The cost of an ambitious international nuclear fusion project has trebled in three years, scientists say, prompting fears the work may be scrapped.

At the same time, financial crises have beset all the nations involved in the project.

As a result, construction of the International Thermonuclear Experiment Reactor (Iter) at Cadarache in France has been pushed back from 2015 to 2019, and more delays are likely.

Some scientists say there is a risk that the entire project could be cancelled. The original price tag was €6 billion. ($11 billion). The latest estimate is €18 billion.

Because it is hoped that fusion plants could one day supply the world with cheap, non-polluting power, the crisis facing Iter is a substantial threat to plans to tackle world energy and climate problems.

Much of Iter's difficulties stem from Europe. The EU - which is struggling to prevent a financial crisis spreading through its member states - was warned last month that it would have to find an extra €1.2 billion to plug a shortfall in construction funds by the end of next year.

An EU memo has called on the 27-member states to "provide the additional resources necessary" for the project, just as these nations are desperately trying to cut their own domestic budgets.

"I think the momentum of the project may be in deep trouble," one Iter scientist told the journal Nature last week. "Time is pressing."

Harnessing the process of nuclear fusion has been a dream since World War II.

Unlike nuclear fission, which powers traditional nuclear reactors and which involves the splitting of uranium atoms, fusion involves combining atoms of hydrogen to create helium, a process that releases vast amounts of energy.

However, fusion occurs only at very high temperatures and huge amounts of electricity are needed first to heat hydrogen isotopes - deuterium and tritium - to 100,000,000C so that fusion can take place.

In addition, a powerful electromagnetic field has be generated to contain the superhot plasma that is created inside a fusion plant.

All prototype fusion plants have consumed more energy than they have generated.

Iter is intended to be the first that will make more power than it uses, and is intended to help scientists design a future generation of plants, each capable of generating as much electricity as a large nuclear reactor, but without producing large amounts of radioactive waste.

Iter's chief executive officer, Kaname Ikeda, says: "It's an exciting project, the fruit of more than 50 years of research."

The Iter project is backed by most major industrial powers, including the US, Europe, Russia, China, South Korea and Japan.

When plans were drawn up for its construction, two prospective sites were chosen, one at Cadarache and one at Rokkasho in Japan. To ensure that the project was built in Europe, the EU pledged to pay the largest slice of its construction - 45 per cent - with the rest being shared among the five others. That move has now come back to haunt Europe.

"We need to build all the major buildings that will house the project and that money is needed now," said a spokesman for Iter.

"When we looked at the detailed design of the project, it was found costs had been badly underestimated. Now we are having to ask EU member states to find extra money at a time when they are having to cope with their own domestic financial problems. Yes, this is crisis, but I am sure the project will still go ahead in the long run."

HOW FUSION WORKS

Nuclear fusion occurs when two nuclei are joined to form a heavier element, releasing enormous amounts of energy.

The reaction takes place on the Sun and other stars, and would be extremely difficult for scientists to control.

Efforts to produce fusion energy have all used more power than they have generated.

- Observer