On the organisation's first anniversary, Natural England chief executive Helen Phillips argued that a review of green belt land should go ahead because the policy has existed for 50 years.
"Nobody can deny that the green belt has achieved its primary purpose of constraining urban sprawl," she said. "But the consequence is that development tends to leapfrog the green belt and more vulnerable land."
Phillips added that the green belt was not intended to deal with today's complex environmental challenges. But she insisted that her comments do not mean that Natural England is "waving a white flag at developers".
A Campaign to Protect Rural England (CPRE) representative argued that such questioning of the green belt's worth would unleash a "torrent of speculation over the future of the countryside that is close to our towns and cities".
The body should instead fight for a strong planning system that offers the "means of securing development within environmental capacity", the group said.
CPRE rural policy head Tom Oliver complained: "We are yet to be convinced that Natural England will be effective in engaging with the threat of a weakened planning system."
He added: "There is also a risk that we would harm wild places with renewable energy infrastructure to save them from climate change."