LONDON, May 30 (Reuters) - Anglo American is starting a formal process to sell its coal assets and explore options for its nickel operations after fending off a takeover threat from bigger rival BHP Group, Chief Executive Duncan Wanblad said on Thursday.

The Anglo CEO faces pressure from investors to boost the company's value by improving operational efficiencies and speeding up the sale of assets after rejecting three takeover proposals from the world No. 1 miner, which on Wednesday said it had no choice but to walk out.

In between approaches and in a bid to stay independent, Anglo laid out its own radical plan to shrink by divesting its steelmaking coal assets, demerging its South African platinum unit, possibly mothballing its nickel mines and selling or demerging diamonds business De Beers. It also said it would slow down work at its fertiliser project in northern England.

"We are now starting the formal processes for the divestment or the demergers of these businesses," Wanblad told Reuters.

"Coal and nickel will be the first and then there'll be the regulatory processes associated with those and then the demerger (of Amplats) and then the De Beers process to follow on completion," he added.

Wanblad said that Anglo's five operating coal mines, development projects and joint ventures in Australia have sparked "more interest" over the past weeks since the divestment plan was laid out. Analysts at Bank of America valued the coal portfolio at between $5.6 billion and $7.9 billion.

A spin-off of Johannesburg-listed Anglo American Platinum (Amplats) could also see a listing in London, Wanblad said, following a similar process to its South African thermal coal assets' spinoff Thungela Resources in 2022.

(Reporting by Clara Denina and Felix Njini; Editing by Susan Fenton)