The move, announced in last week's autumn statement, would place even more importance on how sustainable development was defined in the National Planning Policy Framework, said the CPRE.
Policy director Neil Sinden said the Government had dithered on a concrete definition, prompting people to worry it implied any kind of development. The three agencies often highlighted proposed projects that could be damaging, he added.
"Unless sufficient priority is given to protecting and enhancing the environment, this move could emasculate their roles as watchdogs and do untold damage to England's countryside. Without explicit safeguards, developers could ride roughshod over the views of local people."