Environmental advocacy groups sent a letter to the British government Thursday arguing solar energy could be cheaper than oil with the right policies in place.

"Solar could be cheaper than fossil fuels in just a few years, but it needs a little more help from government to get it there," Friends of Earth campaigner Alasdair Cameron said in a statement. "Failure to invest now will mean a huge missed opportunity for the British economy."

The British Department of Energy and Climate Change outlined its trajectory for subsidies for renewable energy. Long term, it puts more emphasis on what it considers less established sources of renewable energy like wave and tidal power than it does on established technologies like onshore wind and solar power.

Friends of Earth was joined by Greenpeace and the British Solar Trade Association in calling on the British government to do more to support established sources of renewable power.

They argue developers wanting to establish solar power projects that can generate more than 5 megawatts of energy would have to compete against other forms of renewable power, which would add a layer of investment risk.