"We believe our proposal is responsive to the views of AstraZeneca shareholders and provides a sound basis upon which to arrive at recommendable terms for the combination of our two companies."
Pfizer could tap the debt markets to get further cash to seal the acquisition.
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Any potential tie-up would be the biggest foreign takeover of a U.K. company,and has already led to controversy – although AstraZeneca itself is the product of an international merger.
The government is understood to be unlikely to do anything to block the takeover, but wanted to secure some commitment towards investment in U.K. science.
Lord Sainsbury, a former science minister, spoke out in strong terms against the deal to the Financial Times, and said it would be a "devastating blow" for U.K. science.
U.K. politicians are particularly concerned as Pfizer made hefty cuts to its research and development jobs in the U.K. in 2011, and has a reputation for asset stripping in earlier transactions.
There have also been worries that Pfizer wants AstraZeneca to get around U.S. tax rules, rather than for its pipeline.
"Do we really want to be put in the same bracket as the Cayman Islands or Bermuda?" Chuka Umunna, shadow business secretary, of theU.K.'s opposition Labour Party, said to CNBC.
He also called for more stringent takeover rules and warned that AstraZeneca could "potentially used as apawn in a global tax planning game."
There are also concerns about Pfizer's relatively low research and development spend as a proportion of its income, compared to AstraZeneca, and worries that this might mean less investment in research and development in the U.K.
The deal will probably only face competition issues in China, as both companies have leading positions there, said Andrew Baum, pharmaceuticals analyst at Citigroup.
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There are several key factors driving the deal. One is AstraZeneca's pipeline of promising immuno-oncology, a new kind of cancer treatment, drugs. Another is the tax savings Pfizer could make.
The deal has highlighted how the U.K. has effectively made itself a tax haven for pharma through the patent box regulation, which was first introduced under the previous government and cuts income derived from patents, the lifeblood of the pharmaceutical industry, to just 10 percent.
AstraZeneca's share price was flat in London trading, reflecting the market's lukewarm attitude towards Pfizer's new approach.