Soros Fund Management, the money manager set up by "The man who broke the Bank of England" in 1992, George Soros, wrote an open letter opposing the deal.
Signed by Soros chief investment officer Dawn Fitzpatrick, the letter said it was "disappointed" with both the terms of the deal presented and the process undertaken to agree the deal.
Last week Kennedy Wilson announced plans for Kennedy Wilson Europe Real Estate (KWE) to merge with the US corporate in an all-share deal that valued the combined firm at $8.2bn (£6.4bn).
Shareholders in KWE would receive shares in New York-listed Kennedy Wilson, with KWE becoming a subsidiary of the US giant.
Shares in KWE jumped over 14 per cent after the terms were announced.
Fitzpatrick said she agreed with the concept of a corporate shake-up of KWE.
"We agree that there is meaningful value that should be extracted from KWE and would welcome a sale if priced and structured appropriately," Fitzpatrick wrote.
We urge the board of directors to honour their fiduciary duties and conduct a strategic review of all alternatives available to KWE, including a cash sale to unaffiliated third parties and an orderly liquidation of the company over time.
"A more robust strategic review has the potential to attract additional value-enhancing offers and thus allow KWE shareholders to make a more informed decision as to the intrinsic value of their shares."