(Bloomberg) -- The world’s biggest fusion-energy experiment is planning a reboot without one of its most important members, after the UK announced funding for a rival project trying to replicate the sun’s energy on Earth. 

With the International Thermonuclear Experimental Reactor, or ITER, revising its budget and construction timeline following months of delay, the UK announced Wednesday it’s readying £600 million ($749 million) to attract investors willing to build a competing prototype. The UK’s membership in ITER lapsed earlier this year when it dropped out of the European Atomic Energy Community, or Euratom, and efforts to find a compromise solution have stalled. 

While informal talks continue on a collaboration agreement with the UK, ITER spokesman Laban Coblentz said a proposal would need to be agreed by the project’s remaining members: China, the European Union, India, Japan, Russia and South Korea. “Nothing has been proposed,” he said.

ITER was originally scheduled to cost about $5 billion and begin testing in 2020. The budget has ballooned past $22 billion, with no date set for trials. The dizzyingly complex machine being built in southern France pieces together more than a million parts sourced from around the world, with even the slightest anomalies causing months or years of delay. 

The UK was an original ITER member and key scientific developer. The Joint European Torus in Oxford provided critical data and personnel familiar with fusion reactors. John Wood Group Plc won a $193 million contract in 2016 to manage ITER’s construction.  

While those ITER tenders will no longer be available to UK companies, Prime Minister Rishi Sunak’s government is pressing ahead with fusion technology. 

“In the next two weeks, we are set to launch our search for industrial partners in engineering and construction,” said Paul Methven, chief executive officer at UK Industrial Fusion Solutions Ltd., the company appointed to oversee building a prototype plant.  “We will develop an industry that can deliver commercial fusion for decades beyond.”

ITER’s delay means the project could slip behind privately funded companies such as Commonwealth Fusion Systems LLC and Tokamak Energy Ltd.,  which use smaller versions of the same reactor and expect to begin testing prototypes this decade. Fusion is the process that fuels the sun and theoretically could generate limitless clean energy.

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