On Thursday AstraZeneca's largest shareholder ratcheted up the pressure on its board to engage with Pfizer over a possible sale.US private equity group Blacrock, the world´s largest, and which holds an 8% stake in the pharmaceuticals group, is understood to support the Anglo-Swedish outfit´s decision not to recommend its rival´s £55-share offer to its board, Sky News reported. However, the shareholder then wants AstraZeneca to make use of its option of taking the negotiations up again following the three-month hiatus imposed by UK takeover rules. If it fails to do so then Pfizer would be legally forced to forego any further attempts at an acquistion until six months have lapsed, counting from May 26th. AB