It said a weak outlook for oil prices plus risks for BG in Brazil could make the deal “value destructive for Shell shareholders.”
Few investors or analysts have openly challenged the deal’s strategic benefits for the firm.
Shell said it remained confident of winning the vote.
A Shell spokesman said: “We continue to believe we have the broad base of shareholder support we need for the deal to complete.”
Shell has also won the support of Institutional Shareholder Services (ISS), an influential advisory firm, which recommended that Shell shareholders support the deal.