Concerns about the government's commitment to renewable energy mean the UK has fallen on Ernst & Young's Country Attractiveness Indices table.
The system ranks countries based on their renewable energy infrastructure, growth potential of technologies and national renewable energy markets.
Britain has dropped from fifth to sixth position in the latest report.
This is because of speculation the Department for Energy and Climate Change might turn to natural gas as an alternative fuel to renewables.
Ernst & Young's energy and environmental finance leader Ben Warren said: "The Electricity Market Reform needs to deliver the right framework to stimulate investment across all forms of energy generation, including renewables."
Environmental groups are also concerned about the government's potential dependency on gas and say its new Energy Bill lacks clear objectives.
Make sure you're not paying too much for your energy - compare gas and electricity prices now at energyhelpline.com or call 0800 074 0745