Poland could delay building its first nuclear power plants as natural gas, including shale gas, becomes less costly, the prime minister of the central European heavyweight said Tuesday. With a population of 38 million, Poland uses about 14 billion cubic metres of natural gas a year, of which 60 percent is imported from Russia. Poland relies on coal-fired plants -- fuelled mostly by its own plentiful supplies -- for 90 percent of its electricity and is keen to find alternatives which could help it limit greenhouse gas emissions in line with EU limits. In a bid to diversify its gas sources, it is building its first sea terminal for liquefied gas (LNG), gas storage facilities and connecting its distribution system to neighbouring countries. Warsaw had also planned for its first two nuclear plants to come online by 2024 to feed the needs of its growing energy market. \"I\'m not ruling out nuclear in our energy mix, but later than planned,\" Prime Minister Donald Tusk told reporters in Warsaw. \"This is primarily due to the expected growth of natural gas as an energy source, including domestic shale gas,\" he added. EU member Poland will invest 100 billion zloty (23.5 billion euro, $31 billion) in its energy sector by 2020, Tusk said late last year. Forty billion zloty is earmarked for its first nuclear reactors. Three nuclear power giants have expressed interest in building the atomic facility, including the French groups EDF and Areva, and the US and Japanese firms Westinghouse Electric and GE Hitachi Nuclear Energy Americas. Meanwhile, total investment in exploration and development of the shale gas sector in Poland by both domestic and foreign companies could reach 12.5 billion euros. But earlier this year global energy giants ExxonMobil and Marathon oil dropped shale gas exploration here after finding deposits too deep to extract using the conventional method of hydraulic fracturing, or fracking. Optimistic estimates suggest that Poland could have up to 1.92 trillion cubic metres (67.8 trillion cubic feet) of exploitable shale gas deposits, possibly the third largest reserves in Europe after Norway and the Netherlands.