Building development is having a direct effect on heathland birds in the South of England, a wildlife charity said.
Dartford warblers nest on the protected Thames Basin Heaths
The Wildlife Trust has highlighted two sites, home to Dartford warblers and nightjars, that are planned for houses.
It is contacting all councils to say they are obliged by the EU to consider the impact of development on wildlife.
As many as 578,000 homes are to be built in the region by 2026 in Kent, Surrey, Sussex, Berks, Bucks, Hants, Oxfordshire and the Thames Gateway.
The charity's regional conservation policy director Ian Hepburn said the Wildlife Trust was basing its case on a judgement by the European Court of Justice.
He said the judgement in October ruled strategic land use had to have "appropriate assessment".
And he said that before October 2005, there was no clear obligation to assess impact of development on European protected wildlife sites.
Mr Hepburn said: "There is a real danger that we will destroy the countryside that makes the South East such an attractive place to live, work and visit."
He said one area where development was already having an effect was the Thames Basin Heaths special protection area, stretching across Berkshire, Hampshire and Surrey.
The area is protected under EU law but contains two sites identified for development in the South East Plan, Mr Hepburn claimed.
County councils and unitary councils across the region are due to report their findings on the plan to the South East England Regional Assembly by Friday.
The plan's section on biodiversity states that designated sites will have "the highest level of protection", that damage is avoided wherever possible, that any damage is compensated for, and access to important wildlife areas is ensured.