Scientists in Britain said they have smashed a previous record for generating fusion energy, an achievement hailed a "milestone" on the protracted path towards harnessing a power source considered cheap and clean.

Nuclear fusion is the same process that the sun uses to generate heat and proponents believe it could one day help address climate change by providing an abundant, safe and green source of energy.

A team at the Joint European Torus (JET) facility near Oxford in central England generated 59 megajoules of sustained energy during an experiment in December, more than doubling a 1997 record, the UK Atomic Energy Authority said.

The results "are the clearest demonstration worldwide of the potential for fusion energy to deliver safe and sustainable low-carbon energy," the agency added in a statement.

The doughnut-shaped machine used for the experiments is called a tokamak, and the JET site is the largest operational one in the world.

Inside, a tiny amount of fuel comprising deuterium and tritium -- both are isotopes of hydrogen, with deuterium also called heavy hydrogen -- is heated to temperatures ten times hotter than the centre of the sun to create plasma.

This is held in place using superconductor electromagnets as it spins around, fuses and releases tremendous energy as heat.

Fusion is inherently safe in that it cannot start a run-away process.

Pound for pound (gram for gram) it releases nearly four million times more energy than burning coal, oil or gas, and creates virtually no waste.

The results announced today demonstrate the ability to create fusion for five seconds, but longer times will be needed for the process to become viable as a conventional power source.

"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," said Tony Donne of the EUROfusion consortium.

A larger and more advanced version of JET is currently being built in southern France, called ITER, where the Oxford data will prove vital when it comes online, possibly as soon as 2025.

About 350 scientists from EU countries (plus Switzerland, the UK and Ukraine), and more from around the globe, participate in JET experiments each year.

Meanwhile, so-called DEMO fusion power plants to supply electricity to the grid are being developed alongside tokamak research devices such as JET and ITER.

International cooperation on fusion energy has historically been close because, unlike the nuclear fission used in atomic power plants, the technology cannot be weaponised.

The France-based megaproject involves China, the European Union, India, Japan, Korea, Russia and the US.

ITER chief Bernard Bigot hailed the JET's results as the production of energy on a "nearly industrial scale".

Despite dozens of tokamaks being built since they were first invented in Soviet Russia in the 1950s, none has yet managed to produce more energy than is put in.

British Science Minister George Freeman also hailed the "milestone results".

"They are evidence that the ground-breaking research and innovation being done here in the UK, and via collaboration with our partners across Europe, is making fusion power a reality," Mr Freeman said.