A group of Syngenta shareholders said it opposes any sale of the company to China's state-owned ChemChina and called for the ousting of the Swiss agrichemical group's leadership.
In a letter to the Basler Zeitung newspaper, the managing director of the group that says it was speaking after surveying holders of about 10 percent of Syngenta's capital attacked Chairman Michel Demare.
"The board of directors is in a cul de sac from which it cannot exit on its own," Folke Rauscher wrote. "So the only alternative is a comprehensive renewal of the board of directors at the forthcoming annual meeting."
Demare said last week that Syngenta was in talks about a possible merger and was weighing a number of options. He had said last month that the crop chemicals and seeds company was in talks with ChemChina, U.S. seeds giant Monsanto and others.
"One can justifiably ask whether the board has really thought through the consequences of nationalising Syngenta through a sale to a state-owned enterprise of a communist country," Rauscher wrote.
"Anyone who lives in a successful economy marked by liberal values takes a critical view of such a nationalisation."
Syngenta has declined to comment on the status of negotiations.
Having spurned a $47 billion takeover approach from Monsanto last year, Syngenta is under pressure from shareholders to boost its value even as agricultural markets deteriorate.
A takeover of Syngenta by ChemChina would underpin an effort by the Chinese government to boost farming productivity as it seeks to cut reliance on food imports amid limited farm land, a growing population and higher meat consumption.
A group of Chinese investors including ChemChina agreed last week to buy KraussMaffei Group GmbH for 925 million euros ($1.01 billion), in the biggest-ever Chinese investment into Germany.