Chemicals and health care group Bayer is poised to announce the acquisition of U.S. seeds company Monsanto on Wednesday for more than $66 billion, clinching the biggest deal of the year, people familiar with the matter said.

By accepting Bayer's offer, the largest cash acquisition proposal on record, Monsanto is set to give the German company a shot at grabbing the top spot in the fast-consolidating farm supplies industry, combining its crop science business with Monsanto's strength in seeds. It will also set the stage for the deal to be closely scrutinized by antitrust regulators.