Shadow business secretary Chuka Umunna accused the Prime Minister of "cheerleading for a takeover bid".
AstraZeneca's rejection came after a "fourth and final" proposal from Pfizer as a deadline for making a firm offer loomed.
It insisted that the terms undervalued the company and its "exciting prospects" - though a fall in shares yesterday left the British firm's market value nearly £18 billion lower than Pfizer had been prepared to pay.
The US giant's Scottish-born chief executive Ian Read urged the Astra board to engage in "meaningful dialogue", saying the offer represented "compelling and full value" and adding: "The time for constructive engagement is running out."
Prime Minister David Cameron said: "This is a matter for the companies to resolve themselves, the Government quite rightly should be neutral in this." Downing Street, however, confirmed that contact between the Government and both firms was continuing.
Shadow business secretary Chuka Umunna said: "Labour and others have been clear from the outset that if this potential takeover is found not to be in the best interests of British science, jobs and industry it should not proceed. The next Labour government will act to introduce a strengthened public interest test to better protect Britain's science base."