US drugmaker AbbVie raised its offer for Shire to £30.1 billion today, hoping to win over its reluctant target after three earlier offers were rejected.
The latest £51.15 a share cash-and-stock offer is 11% higher than AbbVie's previous proposal of £46.26.
Shire, the London-listed hyperactivity and rare diseases specialist had said AbbVie's previous bid had fundamentally undervalued the company.
AbbVie is eager to buy Shire both to reduce its tax bill by moving its tax base to Britain - a tactic known as inversion - and to diversify its drug portfolio.
The US company currently gets nearly 60% of its revenue from rheumatoid arthritis drug Humira, the world's top-selling medicine, which loses US patent protection in late 2016.
Chief executive Richard Gonzalez has pressed the case for his pursuit of Shire in a series of meetings with shareholders on both sides of the Atlantic, after setting out the strategic rationale for a deal on June 25. AstraZeneca.
AbbVie said it had now met with, or spoken to, a large number of Shire investors, who collectively represented a majority of its shares.
It is the second attempt by a US pharmaceutical company to buy a London-listed rival, after Pfizer's $118 billion pursuit of AstraZeneca failed last month.
AbbVie said the revised offer reflected a substantial sharing of potential synergies between the shareholders of each company, adding that Shire shareholders would own 24% of the enlarged group.
"AbbVie has made a compelling offer to Shire that creates immediate and long-term value to shareholders of both companies. We think its shareholders should strongly encourage the Shire board to engage in constructive dialogue with AbbVie," Gonzalez said in a statement.
AbbVie reiterated that it wanted to move quickly to get a deal, arguing it would create more value from Shire's assets than Shire could do on its own.
Under British takeover rules AbbVie has until July 18 to announce a firm offer for Shire or walk away.
Shire, while founded in Britain, is today managed out of Boston, headquartered in Dublin and has most of its sales in the US, resulting in a minimal business footprint in Britain.
As a result, the potential takeover of the company has not created the political storm that accompanied Pfizer's pursuit of AstraZeneca.