US drugmaker Pfizer today raised its offer for AstraZeneca to £63 billion, but the British company promptly rejected the proposal.
Pfizer's pursuit of AstraZeneca, which would boost the American company's pipeline of cancer drugs, cut its tax bill and create significant cost savings, comes amid a wave of deal-making in the healthcare sector.
AstraZeneca's board said it had "no hesitation" in rejecting the proposals.
It said it believed the proposals "substantially undervalue" the company and were not an adequate basis on which to engage with its suitor.
Pfizer would much prefer an agreed deal with its rival as hostile takeovers usually take longer, require a higher final price and carry more risks because the bidder cannot access the target's books to assess its business.
Today's £50 a share indicative offer follows AstraZeneca's decision to rebuff an earlier proposal that valued it at £58.8 billion, or £46.61 a share.
AstraZeneca shares were trading at around £30 a year ago, but confidence in the company's cancer drug pipeline has built up strongly since then.
Investors had previously said they were looking for at least £50 a share and also wanted more cash in the mix. The latest deal would offer 32% cash and 68% shares, little different from the 30-70 split offered originally.
After the initial £46.61 approach, made in January, was disclosed earlier this week, AstraZeneca said that offer fell "very significantly" short and the small cash component would leave investors exposed to the risks faced by Pfizer inexecuting an ambitious mega-merger.
Some analysts are convinced Pfizer will raise its offer again, not least because it wants to get the deal done before any possible change in US tax rules that might prevent it moving its tax base to Britain.
Pfizer's latest proposal would have seen shareholders receiving, for each AstraZeneca share, 1.845 shares in the combined company and £15.98 in cash.
The takeover, which would be the largest acquisition of a UK company by a foreign business, has stirred political controversy in Britain.
In an attempt to smooth relations with the government, Pfizer chief executive Ian Read wrote to Prime Minister David Cameron, promising to complete a substantial new research centre planned by AstraZeneca in Cambridge and retain a manufacturing plant in Macclesfield.
The Cambridge site, in particular, is viewed as important to the development of the so-called "golden triangle" of Britain's life sciences industry, spanning Oxford, Cambridge and London.
Read also said that 20% of the enlarged group's research and development workforce would be in Britain.
Pfizer's reputation is under a cloud in Britain following a decision three years ago to shut most of its research work at a large R&D centre in Sandwich, southern England, where Viagra wa sinvented, with the loss of nearly 2,000 jobs.