European scientists on Wednesday announced fresh progress in the drive to make nuclear fusion a practical, safe and clean energy source, saying an experiment at a site in England set a record for the amount of fusion energy produced, more than doubling the previous mark.

Researchers said they achieved 59 megajoules of sustained fusion energy - the same process that powers stars including the sun - at a facility in Culham, near Oxford.

Ian Chapman, the head of the UK Atomic Energy Authority, described it as a landmark event that moves researchers closer to conquering one of the biggest challenges in science. It exceeded the previous mark of just under 22 megajoules of total energy achieved in 1997, they said.

Scientists have cautioned that years of work are still needed, and the level of energy achieved so far is modest. The energy produced in the latest experiment, for instance, was enough to boil about 60 kettles of water.

"We're building the knowledge and developing the new technology required to deliver a low carbon, sustainable source of baseload energy that helps protect the planet for future generations," Ian Chapman said. "Our world needs fusion energy."

Nuclear fusion comes from the fusing of two atoms at very high temperatures, which then release energy.

Scientists have been working for decades to try to develop fusion energy as a viable power source. Unlike the burning of fossil fuels or the fission process of existing nuclear power plants, fusion offers the prospect of abundant energy without pollution, radioactive waste or greenhouse gases.

In the experiment, the fusion reactions at the European joint project JET achieved 59 megajoules of energy over a five-second period. Expressed as a unit of power, that comes to just over 11 megawatts averaged over five seconds. The previous record of 22 megajoules was the equivalent of 4.4 megawatts averaged over five seconds.

Tony Donne, programme manager of the EUROfusion group responsible for the research, said the result shows that the scientists are on the right path.

"If we can maintain fusion for five seconds, we can do it for five minutes and then five hours as we scale up our operations in future machines," Donne said.

Sibylle Gunter, scientific director of the Max Planck Institute for Plasma Physics, said the result would help inform the larger-scale ITER experiment in southern France when that project comes online. It is currently under construction. ITER is a fusion research project supported by China, the European Union, India, Japan, South Korea, Russia and the United States.