The British government is selling the Green Investment Bank to Australian bank Macquarie for STG2.3 billion ($A3.9 billion).
Ministers said under the new ownership at least STG3 billion will be invested in the green economy over the next three years.
The bank was launched in 2012 to channel investment into low carbon development.
UK Climate Change and Industry Minister Nick Hurd said: "The Green Investment Bank has been very successful in attracting private capital to the UK's green economy.
"It now makes sense to move it into the private sector where it will be free from the constraints of public sector ownership, allowing it to build further on its success."
Lord Smith of Kelvin, independent chairman of the Green Investment Bank (GIB), said the board had supported the decision to privatise the government-backed bank as the best route to securing its long-term future and growing green impact.
He said: "There is a compelling logic in the world's first green bank joining forces with the world's largest infrastructure investor.
"When we embarked on this process, we were determined to find a new owner who would build on GIB's successful history - an owner who would have access to deep pools of capital, a commitment to expand GIB's activities, and a respect for the unique role GIB has played in the market.
"Macquarie will bring all of this to GIB, along with its own impressive track record of green investments.
"Its vision for the future growth of GIB demonstrates a redoubling of its commitment to a low-carbon economy."
But Doug Parr, policy director at Greenpeace UK, labelled the sale "a disaster".
"At a time when the government should be shoring up low-carbon industry for post-Brexit Britain, they have given away one of our key tools for advancing green technologies.
"The hole left by the Green Investment Bank will slow our transition to a clean energy system, set us back on reaching our climate targets, and mean more of the jobs from new sectors will go elsewhere."
Since it was formed, GIB, set up with money from the taxpayer, has invested in projects ranging from offshore wind farms and council LED street lighting to replacing boiler systems in sheltered housing, hydropower schemes, energy from waste plants and energy efficiency investments in distilleries.
