US government scientists have reportedly made a breakthrough in harnessing the much sought-after fusion energy power source.

It’s believed fusion energy, the same reaction that powers the sun, could one day provide a limitless and cleaner alternative to fossil fuels.

The Financial Times today reported three separate sources confirmed a recent experiment at the federal Lawrence Livermore National Laboratory in California yielded successful results.

The experiment reportedly produced a net gain in energy produced for the first time since observations began back in the 1950s.

The Financial Times said the fusion reaction, “produced about 2.5 megajoules of energy, which was about 120 per cent of the 2.1 megajoules of energy in the lasers, the people with knowledge of the results said, adding that the data was still being analysed”.

The US department of energy told the news outlet it will announce “a major scientific breakthrough” at the Lawrence Livermore National Laboratory on Tuesday (local time, Wednesday NZT).

“Initial diagnostic data suggests another successful experiment at the National Ignition Facility,” the lab said.

“However, the exact yield is still being determined and we can’t confirm that it is over the threshold at this time. That analysis is in process, so publishing the information…before that process is complete would be inaccurate.”

The Financial Times reports the results are being widely shared behind the scenes in the scientific world.