Agrium, the Canadian fertilizer giant, has stepped up the pressure on the CF Industries board to agree a $3.6bn bid, launching an offer for shares in the US group and accusing its board of ignoring shareholder rights.

Agrium said it had opened its offer for CF Industries shareholders, who could opt to be paid for their shares in cash, Agrium stock or a mixture of both.

The move follows CF Industries' rejection last week of the approach in favour of its own bid for US rival Terra Industries.

Agrium chief executive Mike Wilson  said: "CF's refusal to engage in discussions with Agrium left us with no choice but to take out offer directly to CF stockholders."

It urged CF Industries shareholders to tender their shares and "send a message" to the group's board, which last week revealed it was prepared to rework its Terra bid such that it would not require the approval of CF investors.

 "We believe CF's restructuring of its offer for Terra to circumvent a vote by its own stockholders highlights their belief that CF would lose a stockholder vote and demonstrates a disregard for the rights of their stockholders," Mr Wilson said.

He added that Agrium might be prepared to increase its bid should the CF Industries board highlight additional value from the deal.

CF's chief executive, Stephen Wilson, last week dismissed Agrium's bid as "grossly inadequate" and "opportunistic", adding that it was a "transparent" attempt to interfere with his bid for Terra.

Agrium shares stood $1.84, or 5.1%. higher at $37.89 in lunchtime trade in New York, valuing CF Industries stock at $67.69 at the deal's proposed exchange ratio of 1.7866 Agrium shares per CF Industries share.

CF Industries shares stood $1.26, or 1.8%, higher at $69.87.

Terra Industries shares were $0.65, or 2.4%, higher at $27.57.