German energy giants E.ON and RWE have abandoned a £15 billion ($23.7 billion) deal to build a series of nuclear power stations in the UK.

The companies' exit from the Horizon Nuclear power project leaves the UK government's push for a fleet of new nuclear power plants by 2025 in serious doubt.

Following a strategic review, separately carried out by both parent companies, RWE npower and E.ON UK said today that they would not proceed to develop new nuclear power projects in the United Kingdom through the Horizon joint venture, a joint statement by the companies read.

RWE and E.ON will now focus on finding a new owner for Horizon Nuclear Power. Both parties will endeavor to ensure that Horizon's assets and work on development can be taken up quickly by other potential investors.

The partners will now seek a new buyer for the venture.

The firms had already begun work at Wylfa on the Isle of Anglesey, with RWE investing somewhere in the low triple digit millions of euros in the project, according to Reuters.

RWE shares were down 1.27 percent while E.ON traded 1.79 percent lower at 3 pm EST Thursday.

Dr Tony Cocker, Chief Executive of E.ON UK said: E.ON has decided to focus its investment in the UK on other strategic projects that will allow us to deliver earlier benefit for customers and our company, rather than the very long term and large investment new nuclear power calls for.

The firms cited the German government's decision to phase out nuclear power and the high costs associated with Horizon for the decision.

Sources close to the deal told Reuters the pair were also looking for assurances from the government they would see a long-term return on their investment.

It's about timing. The government was not moving fast enough, the source said.