Britain's nuclear expansion plans were boosted on Tuesday after Japanese company Hitachi signed a £700m deal that will enable it to start building the next generation of power plants.

The Japanese energy and engineering company is buying Horizon Nuclear Power, which has the rights to build reactors at Wylfa on Anglesey, north Wales, and Oldbury in Gloucestershire, from its German owners E.ON and RWE npower.

In what it described as the start of a 100-year commitment to the UK, Hitachi confirmed that it intends to progress Horizon's plans to build between two and three new nuclear plants at each site.

The facilities, which could be feeding electricity into the National Grid in the first half of the 2020s, are expected to generate power equivalent to up to 14m homes over 60 years.

Up to 6,000 jobs are expected to be created during construction at each site, with a further 1,000 permanent jobs at both locations once operational.

The prime minister, David Cameron, said: "I am determined that Britain competes and thrives in the global race for investment. This is a decades-long, multibillion 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."

Hitachi has also signed supply chain deals with UK engineering firms Rolls-Royce and Babcock International and has also pledged to establish a module assembly facility in the UK.

It beat off a rival bid for Horizon from a consortium led by US nuclear engineering company Westinghouse.

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 announcement came as it was claimed that renewable energy capacity in the UK will overtake nuclear power capacity by 2018.