Agrium, the Canadian fertilizer giant, put its takeover battle for CF Industries into the endgame by saying it would walk away unless a "compelling majority" of investors in its US rival agree to a deal.

Agrium, which has received the support of holders of just 3.1% of CF investors so far, extended the deadline for investors to accept its $4bn offer by a week, to June 22.

However, it said there would be no further improvements to its terms – unless the CF Industries board, which has refused to enter talks on grounds that Agrium's offer is too low, can demonstrate at the negotiating table why a higher price is justified.

Agrium has twice sweetened its offer twice since making its initial bid in February.

'Clear signal'

Mike Wilson, Agrium's chief executive, urged CF investors to send an "unambiguous message" to their board by tendering their shares to the offer.

"If we receive a compelling majority of shares tendered into our offer, we will continue to press CF to engage," he said.

Agrium would pull out if the offer continued to receive a lukewarm response.

"If we fail to receive a compelling majority, we will conclude we do not have sufficient support from CF stockholders and will walk from the transaction," Mr Wilson said.

He added that 14 out of CF's 15 biggest investors had "dramatically" cut their holdings, "a clear signal that they disagree with CF's views on valuation and are voting with their feet".

Market reaction

The comments came shortly ahead of a statement from CF that it had received a request for more information from regulators looking into its own hostile approach, to US-based Terra Industries.

The request represented a "routine aspect of the regulatory process", CF said.