The Government's consultation into its East of England plans closes today.
According to a report commissioned by Friends of the Earth, the East of England plans could contravene EU laws aimed at protecting Europe's wildlife sites.
The Government has already made changes to its Regional Spatial Strategy (RSS) for rapid growth in the area, but these changes may not go far enough.
The report's author, Professor Riki Therivel, of sustainability consultants Levett-Therivel said: "even with the further proposed changes, the RSS could still lead to significant impacts on the integrity of the European sites."
"The Government's house building plans, particularly for the south and east of England, are affecting wildlife by hitting against environmental limits.
"Some of the problems could be solved by finding other places to put the houses, but in the long run we will need to significantly change how houses are built.
"In some areas we may need 'water neutral' and 'air pollution neutral' homes to stay within environmental limits. These wildlife sites are the jewels in England's nature conservation crown. If we can't keep them in good shape, then what hope have we for our own future?"
According to Friends of the Earth's East of England campaigner, Mary Edwards, "The Government is trying to force too many people, houses and related infrastructure into some of our most environmentally sensitive areas.
"This report warns that not only is the Government's revised East of England Plan still too environmentally damaging, it may also be unlawful. Ministers must urgently reassess these proposals and ensure that our best wildlife sites are properly protected."