By Sinead Cruise

LONDON (Reuters) - Henderson Global Investors, a minority investor in Bayer has demanded a vote on the firm's $64 billion-plus proposed takeover of Monsanto, which it said threatened the long-term strength of the German chemicals group.

Asim Rahman, European equities fund manager at Henderson, one of Bayer's 20 largest investors with a 0.7 percent stake, said the bid for the U.S. seeds company represented "a major departure from a strategy of focus and integration of existing acquisitions" that Bayer had consistently communicated to the market for a number of years.

Bayer announced a sweetened $125-a-share offer for Monsanto, the largest all-cash takeover bid on record, on July 14 in an effort to entice its reluctant management to give the German company full access to its books.

Global agrochemical firms have been weighing a string of big-ticket mergers with rivals in recent months to partly offset a sharp drop in commodities prices that has hit the incomes of some of their biggest agricultural customers.

In a letter to the company dated June 7, Henderson's Rahman also called for a shareholder vote on the deal to restore investor trust and ensure support for Bayer's future strategic direction, which he hoped could minimize a likely increased conglomerate discount on its share price relative to peers.

"We cannot accept the Board's decision to deny shareholders any opportunity to vote on it ... Technically the transaction does not require shareholder approval, but an endorsement by shareholders would provide an opportunity to repair market trust in the investment case," he said.

After meeting with Bayer in May, Rahman said he was unconvinced that the deal would create value for shareholders.

"The acquisition terms reflect paying a very high valuation multiple and the deal could constrain inorganic investment in the pharma division at a time when the future pipeline of this division is a key concern for investors," Rahman said.

"Following a future integration of Monsanto, Bayer could find itself with a weakened pharma business," he added.

Monsanto shares closed at $100.50 on Monday, making Bayer's offer reflect a premium of around 25 percent over current stock values. Shares in Bayer were down 1.4 percent at 91.6 euros at 0900 GMT.

Other minority investors echoed Henderson's caution over Bayer's high-stakes pursuit of Monsanto and flagged fears the German firm could be tempted to bid even higher to secure its prize while rival buyers like BASF wait in the wings.

"I am happy that the new offer made by Bayer did not go as far as $135-$140. However I don't think it will be enough -- Monsanto management rejected the $122 offer as financially inadequate," Andrea Williams, Senior Fund Manager at Royal London Asset Management, told Reuters.

(Editing by Carolyn Cohn and Louise Heavens)