AstraZeneca said it concluded that the proposal "very significantly undervalued AstraZeneca and its prospects."
Another effort to revive discussions took place Saturday, though Pfizer said Astra did not engage. Pfizer remains interested and is confident a combination is possible, issuing a statement to the markets underscoring the potential advantages of the deal.
"Pfizer believes the strategic, business and financial rationale for a transaction is compelling," the company said in a statement.
Shares traded Monday at 46.77 pounds after the approach was revealed.
The deal comes as drugmakers take advantage of lower interest rates in a spate of mergers and acquisitions. One such deal last week involved Swiss drugmaker Novartis AG, which agreed to swap its vaccine business for GlaxoSmithKline PLC's cancer drug unit and sold its veterinary drug arm to Eli Lilly and Co.
Pharmaceutical companies are trying to boost sales and cut costs as many see revenue flat line or even dip due to increased competition from generic drugs and rising expenses for the development of new products.
Pfizer's bid for AstraZeneca would mark the biggest-ever foreign takeover of a British business, and it is possible the Conservative-led government of David Cameron may take a position on the matter, particularly since it has taken an interest in bolstering the country's biotech sectors.
"It would raise fears about the protection of AstraZeneca's innovative research program in the U.K., especially given that Pfizer has already looked at the U.K. and decided to remove its own R&D operations," said Sarah Main, the director of Campaign for Science and Engineering, an independent advocate for science and engineering in the U.K.
AstraZeneca is currently undergoing a major research and development re-organization to offset the expiration on patents for drugs like cholesterol medication, Crestor. The company has been reducing costs and trying to make research programs more productive.