The Ruia family's Essar Global Fund vehicle confirmed at the end of last week it is in talks about making an offer for the 22% stake it does not own in the London-listed oil and gas producer.
Essar Group, controlled by Shashi Ruia and his brother Ravi, raised £1.2 billion from the sale of a stake four years ago. Reports suggest that its offer will be pitched at 75p a share, a fraction of Essar Energy's 420p flotation price.
David Cumming, head of equities at Standard Life Investments, said: "Essar Global's potential attempt to buy out the minority shareholder position in Essar Energy, capitalising on a technically depressed share price, is a calculated attempt to deprive minority shareholders of the substantial future upside in Essar Energy's valuation.
"This potential bid is an example of cynical opportunism and should not be allowed to proceed."
He called on the Essar Energy board to protect minority investors.
SLI is the fourth largest investor in Essar Energy. Its 2.8% stake was worth £24.2 million at Friday's closing price of 66p. Scottish Widows Investment Partnership has a £29.5m holding.