The EU has opened an in-depth investigation into Bayer's proposed $66bn (£51bn) takeover of Monsanto over competition concerns.
The deal between the German chemicals and pharmaceuticals firm and the US seeds group would create the largest integrated pesticide and seed company in the world.
"Seeds and pesticide products are essential for farmers and ultimately consumers," said competition commissioner Margrethe Vestager.
"We need to ensure effective competition so that farmers can have access to innovative products, better quality and also purchase products at competitive prices. And at the same time maintain an environment where companies can innovate and invest in improved products."
The European Commission said it was concerned the takeover would reduce competition in markets including herbicides, vegetable and crop seeds, genetically modified seed characteristics and digital agriculture, especially as it follows two other mega-mergers in the sector.
Bayer's purchase of Monsanto would follow Dow Chemical and DuPont's recent $130bn merger and ChemChina's $44bn takeover of Swiss firm Syngenta, in which the commission intervened to protect competition for the benefit of farmers and consumers in both cases.
Regulators have until 8 January to make a decision on the acquisition, which is the biggest in German corporate history.
Bayer is still confident it will seal the deal by the end of the year, saying it had expected the review and will continue to work closely and constructively with the European Commission on the investigation.
"Bayer looks forward to continuing to work constructively with the commission with a view to obtaining the commission’s approval of the transaction by the end of this year," the firm said in a statement.