Plans for a nuclear power station at Hinkley Point C in Somerset, will "lock a generation of consumers into higher energy bills", Greenpeace said.
Executive director John Sauven said: "It will distort energy policy by displacing newer, cleaner, cheaper technologies.
He added, "With companies now saying the price of offshore wind will drop so much it will be on par with nuclear by 2020, there is no rationale for allowing Hinkley C to proceed.
"Giving it the green light when there is no credible plan for dealing with the waste could also be in breach of the law."
More top news
Southern areas windy and very mild.
Nobel-prize-winning schoolgirl Malala Yousafzai has said she is a proud Brummie, as a new portrait of her was unveiled in her adopted home.
The Filipino girl group lost out to Lauren Murray in the sing off after receiving the lowest number of votes.