FILE PHOTO: Monsanto logo is displayed on a screen where the stock is traded on the floor of the New York Stock Exchange (NYSE) in New York City, U.S., May 9, 2016. REUTERS/Brendan McDermid/File Photo

BRUSSELS (Reuters) - EU antitrust regulators are set to warn Bayer its planned purchase of U.S. seed maker Monsanto may hurt competition, a person familiar with the matter said on Friday, a move that would force Bayer to offer concessions to address the concerns.

The $66 billion deal would make Bayer BAYGn.DE the world's largest pesticides and seeds company, an outcome already facing strong criticism from environmentalists and some farm groups.

The European Commission is expected to send a charge sheet known as a statement of objections to the companies in the coming weeks, but a final decision has not yet been made, the person said.

The EU competition enforcer declined to comment. Companies have to address all the concerns addressed in the document with concessions or face a veto.

“Bayer continues to work with the necessary regulatory parties, including the EU, toward a successful close of the acquisition of Monsanto in early 2018,” Bayer spokesman Darren Wallis said in an email. He declined further comment.

Monsanto spokeswoman Sara Miller said the company had no comment.

Bayer and Monsanto MON.N met with Commission officials earlier this week and were told of the regulatory concerns.

Bayer agreed in October to sell its seed and herbicide units, including its LibertyLink-branded seeds and Liberty herbicide businesses, to world No. 3 crop chemicals maker BASF BASFn.DE for 5.9 billion euros as part of an attempt to ease competition concerns.

It has yet to make a formal offer of concessions to the Commission which opened a full-scale investigation into the deal in August after expressing preliminary concerns that it may reduce competition and jack up prices.