But the government has insisted it remains committed to reducing CO2 emissions by 20% by 2010.
The society published on Monday its response to the current review of the UK climate change programme.
Sir David Wallace, vice president of the Royal Society, said: "There are some tough political decisions to be made, in this parliament, about how the UK manages its seemingly insatiable appetite for energy at a time when cutting emissions of greenhouse gases such as carbon dioxide is imperative.
"This is underlined by the fact that, at the current rate, even the government’s revised assessment of how much carbon dioxide the UK will cut is frankly unrealistic."
The Royal Society warns that unless the rate of development of both renewable and energy efficiency measures make up for the loss of capacity resulting from the phasing out of nuclear power, the UK will actually become more reliant on fossil fuels with the result that carbon dioxide emissions will go up rather than down.
The UK’s emissions of greenhouse gases rose between 2003 and 2004, according to the government’s own data.
The emissions last year were 1.5% above those in 2003, and are now higher than at any time since the Labour government came to power in 1997