A proposed extension to a giant wind farm between Essex and Kent could be blocked over fears it would drive away a species of sea bird.
The area is the winter home of about 6,500 red throated divers - more than a third of the UK's population.
The consortium behind the London Array windfarm wants to build phase two of the project, which will eventually boast 341 turbines.
But the RSPB fear more turbines will disturb the birds.
The wind farm was given the green light in 2006 on condition that phase two would only go ahead if the red throated divers were protected.
The RSPB said the population of the birds in the outer Thames estuary could be displaced by the extension.
Richard Rigg, project director at London Array, said: "Since receiving its original consent in 2006, London Array has been working alongside Natural England and the RSPB to collect, analyse and review information on the red throated divers that winter in the Thames Estuary.