The shareholder rebellion at Bayer AG is poised to worsen after its supervisory board ignored an unprecedented no-confidence vote against the leadership of Chief Executive Officer Werner Baumann.

Several top investors of the German chemicals and drugs giant are frustrated with directors’ decision to back Baumann and his strategy that led to the $63 billion takeover of Monsanto, according to people familiar with the matter. The investors consider the move a sign that Bayer is unwilling to address shareholder concerns, the people said, asking not to be identified because the deliberations are private.

Bayer now needs to consider an overhaul of the oversight board led by Werner Wenning, take a more forthcoming approach in dealing with litigation in the U.S. and conduct a strategic review — including a potential breakup of the conglomerate into crop science and pharmaceutical companies, they said.