FRANKFURT/BRUSSELS (Reuters) - Bayer's BAYGn.DE quest to win regulatory approval for its planned takeover of Monsanto MON.N ran into further delays on Monday as European Union antitrust investigators extended their investigation by five working days until March 12.

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The ongoing proceedings would have given the German drugs and pesticides maker until midnight on Monday to offer further concessions to EU regulators - or request another extension of up to 10 working days.

The EU did not state a reason for the extension to March 12 to deliver its verdict on the merger, which effectively gives Bayer a new deadline at midnight Feb. 5 to propose concessions such as asset sales.

A Bayer spokesman declined to comment, beyond confirming the EU’s statement.

The transaction, opposed by environmentalist groups and by some farmers' associations, would make Bayer the largest supplier of seeds and crop chemicals, surpassing ChemChina's Syngenta, DowDuPont DWDP.N and BASF BASFn.DE.

Late last year, the EU Commission granted Bayer more time to provide information, which pushed the deadline for the EU to decide over the deal back to March 5 from Jan. 22. Bayer had initially hoped to wrap up the deal by end-2017.

Bayer’s Chief Executive Werner Baumann has described antitrust procedures as going to “unimaginable depths”, with more than 4 million pages of documents provided to EU investigators, but the delays have also allowed the target company to cut its debt, lessening the financial burden for Bayer.

The Monsanto deal’s value last year was cut to $63.5 billion including debt, down from an initial $66 billion, because the U.S. seeds giant had lowered its financial liabilities.

Bayer has already agreed to sell certain seed and herbicide businesses for 5.9 billion euro ($7.3 billion) to domestic rival BASF but has said it was bracing for antitrust authorities to demand more asset sales.

($1 = 0.8089 euros)