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PARIS/LONDON, Jan 23 (Reuters) - French utility EDF on Tuesday again pushed back the start date on its long-delayed 3.2-gigawatt (GW) Hinkley Point C reactor plant in Britain to at least 2029, with a new estimated cost of between 31 billion and 34 billion pounds ($43.06 billion) based on 2015 values.

The project in southwest England, Britain's first new nuclear plant in more than two decades, was at the last update expected to start operations in June 2027, with an estimated cost of 25-26 billion pounds, which also was a revision of a previous 2025 start date at a cost then estimated at 18 billion pounds. EDF had initially said it would be powering British homes in 2017.

EDF said that the new target date is based on current productivity goals as the company shifts to the ramp-up of electro-mechanical work, following the installment of the dome on unit one in December.

The new delay comes after the UK government recently announced plans to speed up new nuclear projects by introducing a timeframe requiring an investment decision every five years from 2030 to 2044 and to also explore the potential of another new large-scale nuclear plant.

Britain aims to increase its nuclear energy capacity to 24 GW by mid-century, up from 6 GW today, which could meet around a quarter of the country's forecast electricity demand.

Another 3.2-GW planned plant - Sizewell C in southeast England - has not yet had a final investment decision (FID). The government said on Monday it would invest an extra 1.3 billion pounds in the project.

EVEN FURTHER DELAYS POSSIBLE

For the Hinkley site, EDF said the electro-mechanical work required follows the basic building phase, where the contractor switches from tasks like pouring concrete to wiring and setting up the reactor.

EDF said there are two other possible scenarios: the first seeing a delay until 2030 with the same cost; the second envisages that the project will be postponed until 2031.

"The costs of completing the project are now estimated at between 31 billion and 34 billion pounds in 2015 values. The cost of civil engineering and the longer duration of the electro-mechanical phase...are the two main reasons for this cost revision," EDF said in a statement.

"If the risk of an additional delay of 12 months mentioned above in the final scenario does materialise, it would result in an estimated additional cost of around 1 billion pounds in 2015 values," it added.

Other similar new-build nuclear projects in Flamanville, France, and Olkiluoto, Finland have been delayed and faced big increases in costs.

($1 = 0.7895 pounds) (Reporting by Forrest Crellin and Benjamin Mallet; additional reporting by Nina Chestney, Susanna Twidale and Farouq Suleiman in London; Editing by Jane Merriman and Marguerita Choy)