Shire is believed to be seeking a higher takeover offer from US drugmaker AbbVie Inc that would value the company at about £31bn ($53bn).
Under British takeover rules AbbVie has until July 18 to announce a firm offer for Shire or walk away. AbbVie, which is based in suburban Chicago and makes top-selling arthritis drug Humira and other medicines, raised its cash and stock offer to about $51bn last week, hoping to win over its reluctant target after three earlier offers were rejected.
AbbVie is eager to buy Shire both to reduce its US tax bill by moving its tax base to Britain – a tactic known as inversion – and to diversify its drug portfolio.
AbbVie currently gets nearly 60 per cent of its revenue from rheumatoid arthritis drug Humira, the world’s top-selling medicine, which loses US patent protection in late 2016. In an interview last week AbbVie chief executive Richard Gonzalez refused to be drawn on the possibility of increasing his offer, but said he was leaving all options open.