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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 wind farm 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.

"London Array believes the proposed scheme, which uses only part of the area available for phase two, can maintain the integrity of the Special Protected Area in which the wind farm sits and will not adversely affect the population of red throated divers."

Martin Harper, RSPB conservation director, said: "The RSPB has been striving to ensure the London Array project delivers vital green energy while not putting at risk the red throated divers of the Thames Estuary.

"Our work with the developers in phase two has continued but unfortunately we have not been able to agree with their conclusion that phase two won't harm the red throated diver population."

The RSPB said it would be sending an objection to the lifting of the conditions.

When completed, the London Array will be one of the world's largest offshore wind farms covering an area of 90 sq m (232 sq km) between Margate and Clacton.

The wind farm will have 341 turbines about 12 miles off the Kent and Essex coasts.

Construction of phase one began in July 2009 and is expected to be completed by the end of the year.

When completed, phase one and two will provide enough electricity for over 650,000 homes, London Array said.

Phase two would cover an area of over 16 sq m (40 sq km) along the eastern boundary of the phase one area.