"Capital flowing into renewable energy climbed from 80 billion dollars in 2005 to a record 100 billion dollars in 2006," the United Nations Environment Programme (UNEP) said in a statement.

The report, entitled "Global Trends in Sustainable Energy Investments, 2007", added that the sector's growth showed no sign of abating this year.

"While renewable sources today produce about two percent of the world's energy, they now account for about 18 percent of world investment in power generation, with wind generation at the investment forefront," UNEP said.

"Solar and bio-fuel energy technologies grew even more quickly than wind, but from a smaller base," it added.

The report said sustainable energy investments grew 43 percent from the previous year in 2006 to reach 70.9 billion dollars, while another 30 billion entered the sector over the same period via mergers and acquisitions, leveraged buyouts and asset refinancing.

"Sustainable energy markets are becoming more liquid and more global. The various forms of capital now being deployed across the value chain signal the sector's shift into the mainstream," the 48-page report said.

"Renewable energy and energy efficiency are no longer niche sectors that are promoted only by governments and environmentalists," it added.

The report, which was prepared in cooperation with New Energy Finance Ltd, a specialised research and analysis firm, offered three main reasons for the revewable energy sector's boom.

"Greening of industry and public awareness of climate change and other environmental issues are key drivers of renewable energy and energy efficiency," the report said.