* To sell 150 MW Ormonde wind park in Irish Sea - sources
* Could fetch 400-500 mln pounds - source
* Sale likely to start next year - source
* Move part of an overhaul Vattenfall announced in July
LONDON/FRANKFURT, Nov 25 (Reuters) - Vattenfall has mandated Morgan Stanley to organise the sale of a British wind park as the Swedish utility scales back outside its home market, two people familiar with the matter said.
Vattenfall has invested 2.3 billion euros ($3.1 billion) in six UK wind parks since 2008.
The first power generating asset to come on the block will be Vattenfall's Project Ormonde, a 150 megawatt facility in the Irish Sea, the sources familiar with that transaction said. It could fetch around 400-500 million pounds ($650-$810 million), one of them said.
"The sale will likely start some time next year, as Vattenfall is waiting for clarity on some regulatory issues, which are crucial to determine the profitability and price of the assets," one of the sources said.
A spokesman for Vattenfall said a reorganisation announced in July would allow it to respond to tough European market conditions, but it was "too early to say how Vattenfall will go about optimising the value of its portfolio."
Morgan Stanley declined to comment.
Utility sector bankers expect Vattenfall to sell the remaining UK wind assets later with proceeds likely to at least match its initial investment.
Scandinavia's biggest utility is under pressure to pull back to its better-performing home markets after acquisitions that saddled it with 158 billion Swedish crowns ($24 billion) in debt and led to major writedowns.
It said in July it would split into two units - one Swedish, one for the rest of Europe - by the end of 2013, in what was seen as a first step towards divesting the latter.
Vattenfall's UK wind business has a total capacity of around 612 megawatts (MW), including the Thanet offshore wind farm, one of the largest offshore wind parks in the world at 300 MW installed capacity.
Vattenfall also has a large pipeline of planned onshore and offshore wind projects in Britain, including the European Offshore Wind Deployment Centre, a testing site for new types of turbines.
Utilities, infrastructure investors and pension funds were likely to take a look at the Vattenfall assets, the sources said.
While investors seek returns on equity of 10 to 12 percent for projects like Vattenfall's Ormonde, the profitability hinges on the regulatory framework.
Britain gives direct subsidy support to renewable energy projects, including wind farms. However, it is currently changing its mechanism for awarding subsidy payments and the legislative framework to support these changes is not expected to be announced before Spring 2014. That is likely to leave some investors wary.