The Perth-based company said Npower renewables, the UK renewables business of Germany's RWE, was paying £308 million for the stake, a price that included capital expenditure undertaken so far.

Greater Gabbard was created as a joint venture between Airtricity, the Irish wind farm company, and US engineering company Fluor.

SSE, now Scotland's second-largest company by market capitalisation, acquired half of the project when it bought Airtricity for £1.1 billion in February.

It gained outright control of the project in May, paying £40m to Fluor, which sold the stake after securing the contract to construct the wind farm.

Under the deal, RWE will reimburse SSE for 50 per cent of the capital costs it has already incurred in the project.

A spokesman for SSE refused to give details on how much the company had spent so far, ahead of its interim results announcement later this month.

The spokesman said the sale was designed to "spread the risk" of the project. The total cost of the wind farm is expected to be about £1.3bn.

Analysts had speculated that the company would struggle to find a willing partner to take on half of the project in the current economic climate.

SSE will act as the operator of the project under a management services agreement, which covers both the construction and operational phases.

Onshore construction has already started, with offshore work expected to begin in the second half of 2009, with the first power being generated in 2010 and final completion in 2011.

Ian Marchant, chief executive of SSE, welcomed RWE's involvement, saying its long-term involvement in offshore developments would complement his company's expertise in major renewable projects.

"This year has been characterised by high and volatile prices for fossil fuels and by political uncertainty in key oil and gas producing regions," Marchant said. "All of this again demonstrates the long-term value of energy sources which are both renewable and indigenous."

Marchant said Greater Gabbard was "a major development in every sense" and the company was focused on ensuring smooth and timely construction "so that it can begin to play its important part in meeting the UK's energy needs as soon as possible".

Located east of Felixstowe in the Thames estuary, the site wil have up to 140 turbines e built on two sandbanks about 12 miles offshore.

By the time it is completed in 2011, the wind farm is expected to generate up to 500 megawatts, enough to power about 415,000 homes.