MILAN, Nov 29 (Reuters) - Italian energy company Eni (ENI.MI) is ready to invest more in a nuclear fusion project it feels could be a game-changer in the race to produce limitless clean electricity to power cities and industry, the head of the project at Eni said. In 2018 Eni injected $50 million, as starting investment, to become the single biggest shareholder in Commonwealth Fusion System (CFS), a firm set up by the Massachusetts Institute of Technology to produce energy by fusing atoms at temperatures as hot as the sun."CFS is in a new round of financing for its next 2025 deadline and we"re watching things carefully. We are very interested in fusion and intend to build the business," head of Eni"s Fusion Program Francesca Ferrazza told Reuters. Earlier this year CFS met a key milestone to building a fusion power plant by successfully testing the coils and magnets that will hold the super-hot plasma of subatomic particles.It is now talking to investors to help it fund the next deadline - to build a pilot reactor by 2025 that can generate more energy than it consumes. As governments round the world pick up efforts to combat climate change, energy companies like Eni have launched transition strategies to shift from fossil fuels into cleaner energy. Fusion, once considered science fiction, is now attracting interest from big companies. Norway’s oil and gas group Equinor (EQNR.OL) this year joined CFS and U.S. major Chevron Corp (CVX.N) has invested in Seattle-based fusion startup Zap Energy Inc. "Once fusion was the domain of scientists but now there are private investors interested and what they want to see is a shorter timeline to exploitation," Ferrazza said. CFS expects to have a first commercial plant ready to start feeding energy into the power grid by the early 2030s. The first-of-its-kind plant, which will be built in the United States near Boston, could cost up to around $3 billion. "There’s a whole industry to be built – it’s like the solar business 30 years ago... it took time to create economies of scale and the supply chain," Ferrazza says. Further out CFS aims to build 200-400 megawatt plants and ship fusion technology to the world, using 3D printing for some of the critical parts. According to Kepler Chevreux it may not be unrealistic to imagine 2,000 gigawatts of nuclear power deployed from 2035 to 2055 using disruptive fusion technology.