The decision by the board of AstraZeneca to oppose what would be the largest-ever foreign acquisition of a British company was backed by Aberdeen Asset Management, which said Pfizer could have done better.
Pfizer has said the latest offer was final but there was speculation yesterday it may return to the fray.
With 2.5 per cent of AstraZeneca's shares, Aberdeen Asset Management could have a significant influence on events.
The asset manager's chief investment officer Anne Richards said the latest £55 per share offer from Pfizer was better than the £53.50 approach AstraZeneca spurned last week but certainly wasn't a knock out.
"Valuing the pipeline of future products in a pharmaceutical company is notoriously tricky" said Ms Richards, adding: "Takeovers in the pharmaceutical sector can be disruptive to research and development and don't always increase shareholder value."
She noted shareholders would be paid more than half the takeover price in Pfizer shares, listed in the US.
AstraZeneca chairman Leif Johansson said he had told Pfizer in discussions that his board could only recommend a bid that was at least 10 per cent above an offer of £53.50 pounds made by Pfizer on Friday, or £58.85.
Pfizer wants to create the world's largest drugs firm, with a headquarters in New York but a tax base in Britain, where corporation tax rates are lower than in the United States.
The plan has met entrenched opposition from AstraZeneca, as well as politicians and scientists who fear cuts to jobs and research.
Mr Johansson said Pfizer's approach had been driven by the corporate financial benefits to its shareholders of cost savings and tax minimisation.
Aberdeen Asset Management's chief executive Martin Gilbert said recently the proposed deal raised public interest issues.
But, some big shareholders appeared unhappy yesterday, when Astrazeneca's share price fell 11 per cent, £5.36p to £42.88.
Alastair Gunn of Jupiter Fund Management said: "We are disappointed the board of AstraZeneca has rejected Pfizer's latest offer so categorically. They should have at least engaged in a constructive conversation with Pfizer."
The firm has until May 26 to increase its offer under UK takeover rules.
Mr Johanssen said: " I think the language is very clear in Pfizer's letter that this is their absolute last and final (offer). I think the (UK) Takeover Panel might hold them to that."
Ms Richards said: "We wouldn't be surprised if they (Pfizer) came back at some point."