Mike Clancy, general secretary-designate of the Prospect union, said:
The successful bid by the Hitachi/SNC-Lavalin consortium sees a new entrant to the UK nuclear industry and demonstrates its faith in the economic promise the UK nuclear market offers both commerce and the economy as a whole.
The Horizon venture is an important milestone in securing future low-carbon energy generation capacity within the UK and its importance to local and national economies cannot be overstated.
While Hitachi's advanced boiling water reactor design has yet to undergo the UK's generic design assessment approval process, it is a proven technology and therefore any construction in the UK will benefit from lessons learned from its construction in Japan.
The Horizon venture, which currently employs around 90 people, was set up in 2009 as part of the drive to meet the UK's carbon reduction goals.
But RWE and E.ON put the business up for sale in March after Germany's move to abandon nuclear power in the wake of Japan's Fukushima disaster.
Doubts have grown about the private sector's commitment to the UK's nuclear programme after a consortium made up of Iberdrola, GDF Suez and SSE also withdrew from the process.
David Cameron has welcomed Hitachi's takeover of Horizon Nuclear Power and the Japanese technology firm's commitment to build four to six new nuclear plants in the UK.
This is a decades-long, multi-billion pound vote of confidence in the UK, that will contribute vital new infrastructure to power our economy.
It will support up to 12,000 jobs during construction and thousands more permanent highly skilled roles once the new power plants are operational, as well as stimulating exciting new industrial investments in the UK’s nuclear supply chain.
I warmly welcome Hitachi as a major new player in the UK energy sector.
Japanese technology firm Hitachi has confirmed that it intends to pursue plans to build between four and six new nuclear plants in the UK after acquiring Horizon Nuclear Power.
The new plants, which will be in Wylfa on Anglesey and Oldbury in Gloucestershire, are expected to create up to 12,000 new jobs in the construction phase and 2,000 permanent positions thereafter.
The new infrastructure could generate power equivalent to up to 14 million homes over 60 years.
Rolls-Royce has announced that it will support Hitachi in the delivery of the new nuclear reactors.