AstraZeneca spurned a new £63 billion takeover offer from US rival Pfizer today, saying it "substantially" undervalued the business.

The UK firm rejected the latest advance from the Viagra maker hours after Pfizer stepped up its pursuit by making the increased offer and giving a direct pledge to the Prime Minister on the future of British jobs and research.

At £50 a share, it represented a 7% hike on a previous proposal in January.

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But Astra said: "The financial and other terms described in the proposal are inadequate, substantially undervalue AstraZeneca and are not a basis on which to engage with Pfizer."