M&G Real Estate has let 1 Forbury Place in Reading in its entirety to SSE.
The energy company has taken all 186,000 sq ft in the eight-storey building, which is due for imminent completion.
M&G said that it was the largest single letting in Reading in 25 years and the biggest in the Thames Valley in more than a decade.
Number 1 Forbury Place is part of a three-building complex being developed by M&G RE and Bell Hammer that will provide 600,000 sq ft of prime office space in the centre of Reading.
To date M&G RE has invested more than £200m into central Reading including its acquisition of One Reading Central, a 220,00 sq ft prime office scheme.
M&G and development partner Bell Hammer were advised by Campbell Gordon; SSE was represented by Lambert Smith Hampton.