A leading Syngenta investor has criticized the board for excluding all but "a very small group" of shareholders from talks that could determine the fate of a $45 billion takeover bid by Monsanto, a letter seen by Reuters showed.

Henderson Global Investors, Syngenta's 20th biggest shareholder according to Thomson Reuters data, said the Swiss pesticide maker had relied on a YouTube video to communicate with other investors.

It said this strategy was "short-sighted" and likely to prove "counter-productive" in its attempt to thwart its U.S. suitor.

"The most recent proposal by Monsanto is credible and deserves serious consideration. We have met with Monsanto management to discuss their proposals and we have also engaged with other shareholders," it said in the letter addressed to Syngenta Chairman Michel Demare.

"While we have not yet taken a firm position to support them, the lack of any opportunity to engage with Syngenta is likely to be an important factor in our decision making."

Henderson said it had historically enjoyed good relations with Syngenta management.

"We were therefore very surprised to learn that the company is currently limiting shareholder engagement to a very small group of shareholders, and relying on a YouTube video to communicate with the rest of the investor base," it said.

A spokeswoman for Henderson confirmed the content of the letter to Demare, adding: "We believe the long-term Monsanto strategy is credible for Syngenta stakeholders."

Syngenta could not be immediately reached for comment.

Monsanto, the world's largest seed company, has been hosting meetings with Syngenta shareholders to win support for its bid.