(Reuters) - Britain could need as much as 40 billion pounds ($49 billion) for the planned Sizewell C nuclear power plant in southeast England, nearly double the developer EDF's initial estimate, the Financial Times reported on Tuesday.

New nuclear plants to replace its ageing fleet are key to Britain's efforts to meet climate targets and boost energy security. Sizewell C would generate enough electricity to power around 6 million homes.

The FT report cited one senior government figure and two well-placed industry sources whom it said called the revised figure a reasonable assumption in terms of 2025 prices.

A spokesperson for Britain's Department for Energy Security and Net Zero called the figure "speculative", adding that "discussions with investors are ongoing and commercially sensitive".

EDF declined to comment on the FT report. A spokesperson for Sizewell C did not immediately respond to a Reuters request for comment.

Separately, France's top audit body said on Tuesday that EDF should secure new investors in the project before committing financing.

Sizewell C would be only the second new nuclear plant built in Britain in more than two decades. EDF's Hinkley Point C is expected to start operations in 2029, having experienced several delays and cost overruns.

A year ago, EDF raised the cost estimate of Hinkley Point C to between 31 billion and 34 billion pounds at 2015 prices, up from 25 billion to 26 billion pounds in a prior estimate.

EDF initially said Sizewell C would be around 20% cheaper than Hinkley C. In 2020, Sizewell C was expected to cost around 20 billion pounds.

($1 = 0.8195 pounds)

(Reporting by Kanjyik Ghosh; Additional reporting by Bipasha Dey, Anusha Shah and Rishabh Jaiswal; Editing by Louise Heavens and Mark Potter)