Shire Pharmaceuticals has rejected a STG27 billion ($A49.28 billion) informal bid from US giant AbbVie, arguing that it "fundamentally" undervalued the group, in the latest blockbuster deal to hit the global drugs sector.
The company, which is listed in London and New York but based in Dublin, also highlighted concerns over AbbVie's proposal to make Britain its home base for tax purposes.
The US firm earlier revealed it had made a third cash-and-shares bid, pitched at STG46.26 per share, valuing Shire at the equivalent of $US46 billion ($A49.77 billion).
"The board of Shire decided unanimously to reject the proposal on the basis that it fundamentally undervalued the company and its prospects," said Shire, which is a specialist in the treatment of rare diseases.
It added that its board was also concerned about the proposed structure of the operation "as AbbVie would re-domicile in the UK for tax purposes".
AbbVie, which is listed in New York and was spun off from Abbott Laboratories in January 2013, said it had made an initial approach in early May worth STG39.50 per Shire share.
The blockbuster takeover is the latest multibillion-dollar deal to rock the global pharmaceuticals sector, and comes after Anglo-Swedish group AstraZeneca fought off a $US117-billion takeover bid from US giant Pfizer last month.
Pfizer scrapped its AstraZeneca bid on May 27, ending to a long-running saga that drew widespread attention over concerns that British jobs and research capability would be lost.
There were also accusations that the tie-up was a cynical ploy by Pfizer to avoid paying tax on profits if they were sent to the US. Pfizer had also proposed to switch its tax base to Britain from the US.
AbbVie had meanwhile revealed its takeover approach earlier on Friday following media reports over the move.
"AbbVie Inc notes the recent press speculation and confirms it made an indicative approach to Shire plc with a merger proposal which was rejected by the board of Shire," AbbVie said in a statement to the London Stock Exchange on Friday.
"There can be no certainty that any firm offer will be made. Discussions are no longer ongoing."
Shire responded on Friday that new management had overhauled its performance over the last year - and the AbbVie bid would not allow shareholders to benefit.
"The (AbbVie) proposal fundamentally undervalued Shire and its prospects as a leader in rare diseases and specialty markets," Shire said.
"New management has achieved a step-change in Shire's performance, resulting in significantly accelerated growth and increased shareholder returns over the last 12 months.
"Shire will more than double its 2013 annual product sales to $US10 billion by 2020. The proposal would deny Shire shareholders the full benefits of Shire's growth strategy."
Chairman Susan Kilsby added that shareholders were "strongly advised to take no action in relation to the proposal".
In reaction to Friday's news, Shire's share price surged as high as STG43.18 on the London stock market.
It later stood at STG41.36, up 10.65 per cent from Thursday's closing level.
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