BlackRock's CEO Larry Fink has faced calls to resign from a UK activist investor over the fund giant's approach to ESG investing.

Bluebell Capital Partners, in a public letter, accused Fink of "apparent hypocrisy" for switching positions on thermal coal several times, according to the correspondence seen by International Investment's sister brand Investment Week.

It also criticised him for failing to adhere to BlackRock's own public commitments on sustainability.

BlackRock was in particular attacked by Bluebell for failing to support the activist investor's position on environmental shareholder resolutions at the mining and commodities group Glencore and chemicals group Solvay.

Bluebell has about a 0.01% stake in BlackRock, according to the Financial Times, which has a market capitalisation of $107bn.

Giuseppe Bivona and Marco Taricco, Bluebell's co-chief investment officers, wrote to Fink stating: "As shareholder in BlackRock, we are increasingly concerned about the reputational risk (including greenwashing risk) to which you have unreasonably exposed the company potentially fuelling a gap between the ‘talk' and the ‘walk' on ESG investing".

The letter went on to say that the duo were also concerned about "the backlash caused by BlackRock's ESG strategy which has alienated clients and attracted an undesired level of negative publicity".

It added: "The contradictions and apparent hypocrisy of BlackRock's actions have... politicised the ESG debate".

Elsewhere the letter said: "The reputational damage of being dragged into this politically charged debate, in our view, is very significant because it calls into question the independence of BlackRock as an asset manager."

The Bluebell partners added that it had "direct experience . . . [with] BlackRock's inconsistent approach", referring to the Glencore example.

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BlackRock in response said BlueBell was not necessarily acting in the best interests of investors in its view.

The fund house told the Financial Times: "In the past 18 months, Bluebell has waged a number of campaigns to promote their climate and governance agenda. 

"BlackRock Investment Stewardship did not support their campaigns as we did not consider them to be in the best economic interests of our clients."

BlackRock faced criticisms from both sides of the ESG debate on its home turf in the US, with Republican officials have accused it of being anti-fossil fuel while Democratic officials have criticised it for not backing carbon reduction initiatives enough.