UPDATE 2-GDF Suez joins Iberdrola, SSE in UK nuclear plans

* Three companies to bid for UK nuclear sites

* Iberdrola, GDF agree 50/50 industrial partnership

* Follows previous Iberdrola, SSE agreement

* 3-way partnership to be split 40/40/20

(Recasts with Gaz de France, Iberdrola statement)

MADRID/PARIS, Feb 4 (Reuters) - GDF Suez (GSZ.PA), Europe's biggest utilities provider, has joined Spain's Iberdrola and Scottish and Southern Energy (SSE) in a bid to build the UK's next generation of nuclear reactors.

GDF Suez and Iberdrola (IBE.MC) said in a statement on Wednesday they had formed a 50/50 industrial partnership to bid for development sites currently owned by the Nuclear Decommissioning Authority (NDA) and EDF Development Company.

The news comes two weeks after Iberdrola and SSE (SSE.L) agreed a joint venture to bid for the sites, and will see GDF Suez and Iberdrola each take a 40 percent stake in the venture, with SSE taking the remaining 20 percent.

The statement by GDF and Iberdrola confirmed what a source close to Spain's biggest utility Iberdrola told Reuters earlier on Wednesday.

"Iberdrola has reached an agreement with GDF Suez to develop an alliance on nuclear issues," the source said.

Iberdrola, which part-owns seven nuclear plants in Spain, and SSE said two weeks ago they would form a consortium to build the next generation of UK reactors to replace its ageing fleet of state-built plants.

"This cooperation takes place within the framework of the UK nuclear new build programme whose objective is both to increase development of new nuclear plants and competition in the country," GDF Suez and Iberdrola said in a joint-statement.

Iberdrola and SSE said on Jan. 20 their initial aim was to secure sites suitable for nuclear plants. Under a British government deadline, they have until March 31 to find suitable sites. [ID:nLK563282]

French nuclear energy giant EDF (EDF.PA), which is buying the owner of most of Britain's existing nuclear power plants, British Energy, has said it plans to build two reactors at Sizewell in Suffolk and two at Hinkley Point in Somerset.

It also plans to nominate sites at Heysham in Lancashire, Hartlepool in northeast England and Dungeness in Kent.

German groups RWE (RWEG.DE) and E.ON EONG.DE have already joined forces to compete for the right to build reactors. (Reporting by Astrid Wendlandt and Clara Vilar; Editing by Marcel Michelson and Simon Jessop)