Legal & General Investment Management (LGIM) had already publicly criticised the move, which was announced earlier this month.
However following a meeting with the retailer's deputy chairman Sir David Michels and current chairman Lord Burns, the firm issued a statement on behalf of LGIM head of equities Mark Burgess today rejecting M&S's reasoning for the move.
Burgess said: "While we understand their arguments we do not agree that this is a necessary structure to allow for the successful appointment of a successor to the current CEO. We do not support a dilution in corporate governance standards, particularly in leading UK companies."
An M&S spokeswoman said: "We have made our position very clear over the past couple of weeks and we have nothing further to add."