En+ owns assets in metals and energy, including a 48 percent stake in Hong Kong-listed Russian aluminium producer Rusal, which is a big consumer of hydroelectricity produced by companies owned by En+.

Preparation for an intention-to-float announcement, which usually starts the public marketing phase of an IPO, is well under way, one of the sources said, adding that a delay was still possible, subject to final decision.

En+ declined to comment.

Sources told Reuters in April that the initial valuation of the business would be up to $10 billion, a level some in the industry have said is too ambitious.

En+ aims to raise about $1.5 billion from the possible IPO in London, Deripaska said in June.

This is now more likely to be $1 billion, two of the sources said.

China's CEFC is considering investing in En+ as part of the IPO, industry sources told Reuters this month.

(The story has been refiled to clarify the possible date of IPO launch in the headline.)

(Reporting by Clara Denina in London, Katya Golubkova and Polina Devitt in Moscow; Additional reporting by Anastasia Lyrchikova; Writing by Polina Devitt; Editing by David Goodman)