By Susanna Twidale

LONDON (Reuters) - The developers of a project to expand the world's largest offshore wind farm, Britain's London Array, said a requirement for a study of its impact on birds was forcing them to scrap the plan.

It would take at least three years to complete a necessary assessment of the potential impact to birdlife from the expansion, the consortium of Dong Energy, E.ON and Abu Dhabi state-owned energy investor Masdar said on Wednesday.

The project would have added 200 megawatts of capacity to the existing 630 MW wind farm in the Thames Estuary.

The decision to scrap it was due to "technical challenges and the environmental uncertainties surrounding the site", the group said.

"There is no guarantee at the end of three years that we will be able to satisfy the authorities that any impact on the birds would be acceptable," Mike O'Hare, general manager of the London Array, said in a statement.

The British government aims to install 10 gigawatts of offshore wind capacity by 2020 to help meet emissions reduction goals and European renewable energy targets.

But in the past few months, RWE and Iberdrola-owned Scottish Power have both scrapped or scaled back huge offshore wind farm projects, citing the costs involved in developing deepwater sites.

"The overall project pipeline for UK offshore wind is still healthy, although obviously it's disappointing when projects don't go ahead or are scaled back," said Nick Medic, lobby group RenewableUK's director of offshore renewables.

"We're maintaining our global lead, with more capacity installed in UK waters than the rest of the world put together," he added.

Britain has 22 operational offshore wind farms with 3,653 MW of capacity and a further 23 in the pipeline capable of generating 16,500 MW, RenewableUK said.

(Editing by Nina Chestney and Jane Baird)