UPDATE 1-EU regulators to warn Bayer about Monsanto bid - source

(Adds European Commission declines comment)

BRUSSELS, Dec 8 (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 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 and Monsanto met with Commission officials earlier this week and were told of the regulatory worries.

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 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. (Reporting by Foo Yun Chee; Editing by Alastair Macdonald and Mark Potter)