Last week, Xstrata's second-largest shareholder, Qatar Holding, said it supported the principle of "Glenstrata", but would not support the merger in its current form.
Qatar has been building up its Xstrata stake since the deal was announced in February and last Wednesday increased its holding from 11% to 11.7%.
Glencore is currently offering 2.8 shares for every one Xstrata share held, which Qatar said was insufficient.
In June, Qatar asked Glencore for 3.25 shares for every one Xstrata share. As a result, Qatar said it would vote against the deal in respect of its entire shareholding.
"QH believes that Xstrata has a strong future, whether in combination with Glencore on acceptable terms or as a stand-alone entity, and that its shares represent an attractive long-term investment," Qatar said, adding that it wanted to retain the ability to continue to stock up on Xstrata shares.
The Financial Times has reported that 3% shareholder Norges Bank Investment Bank was also opposed to the deal, while Dow Jones Newswires said shareholder Schroders would also vote down the merger.
Meanwhile, Bloomberg reported yesterday that Sanford C Bernstein said the offer undervalued Xstrata.
"Our valuation puts a fair exchange at 3.16 times versus the 2.8 times on offer," Bernstein senior analyst Paul Gait reportedly wrote yesterday.
Shareholder meetings for both companies will be held tonight.