The companies said Centrica, which operates British Gas, will pay ¬£2.3 billion, or about $3.5 billion, for one-fifth of British Energy, which E.D.F. acquired in January.
The companies said the price represents a discount of about 6 percent for the stake compared to the price of ¬£12.2 billion that E.D.F., Europe¬ís largest electrical utility, paid for British Energy.
As part of the deal, E.D.F. will acquire Centrica¬ís 51 percent stake in the Belgian utility S.P.E. for ¬£1.2 billion. As a result, the cash payment by Centrica is only ¬£1.1 billion.
British Energy operates eight nuclear power stations in Britain. E.D.F. and Centrica said they would also form a joint venture, with E.D.F. holding 80 percent and Centrica the rest, to build four new nuclear power stations there.
The new plants will be European Pressurized Reactors, a type of reactor designed and built by Areva, the French engineering giant. The new reactors are meant to be safer than previous models, incorporating features to prevent core meltdown and protect against exceptional hazards, such as airplane crashes and earthquakes.
Pierre Gadonneix, the E.D.F. chief executive, said in a statement that the deal was a way for Centrica and E.D.F. ¬ďto take part in the relaunch of nuclear energy in the United Kingdom.¬Ē The French state holds about an 85 percent stake in E.D.F.
The discounted price suggests that the value of British Energy has fallen as electricity prices decline in the face of a weakening economy. While that raises the possibility that E.D.F. will have to write off part of its purchase price, Mr. Gadonneix said in a conference call that the discount was ¬ďquasi symbolic.¬Ē
The companies agreed that E.D.F. would operate British Energy, while Centrica will have representation on the board. They will share the power output from British Energy proportional to their ownership, and Centrica will receive 18 terawatt hours more of power at market prices over five years beginning in 2011.
In the statement, Roger Carr, the Centrica chairman, said the deal was ¬ďgood value for Centrica shareholders, improves the strategic balance of our business and further underpins our green energy credentials.¬Ē He also suggested that Centrica was looking for acquisitions in Britain and North America.
The deal is contingent on regulatory approval, and Centrica shareholders will have to back it at an extraordinary general meeting next month.
Shares of Centrica rose 3.6 percent in London early trading, while shares of E.D.F. fell 2.6 percent.