The deal reduces Dong's stake to 25 percent and leaves Germany's E.ON as the project's largest shareholder with 30 percent.

The 630-megawatt London Array offshore windfarm is located 20 kilometres off the coast of Kent and was officially opened in July 2013.

"This is an opportunity for us to invest, alongside established partners, in a quality asset in a growth-driven sector," said Macky Tall, senior vice-president of infrastructure at La Caisse.

La Caisse is one of Canada's largest institutional fund managers. It looks after public and private pension and insurance funds from Quebec with a combined value of around $186 billion.

Britain's renewable energy sector has become an attractive market for investors seeking regulated assets as the government backs the development of green energy projects with long-term subsidy payments.

(Reporting by Karolin Schaps; editing by Jason Neely)