AstraZeneca chairman Leif Johansson has said he believes Pfizer's £69 billion takeover bid is their final offer.
"They say it is their final bid so I have to believe them," he told BBC Radio 4's Today programme.
AstraZeneca has urged its shareholders to reject the offer.
Mr Johansson said the Pfizer bidding had been "controversial from the start" but it was up to shareholders to decide on its fate.
He said the Pfizer offer was "too low", would prove "too disruptive" to Astra's ability to deliver drugs to the market and presented too much of a "risk".