Drugs giant Pfizer tonight stepped up its pursuit of UK rival AstraZeneca by raising the value of its proposed takeover bid to more than £69 billion.
The US company said the offer worth £55 a share was 15% more than its previous proposal on May 2 and also represented its fourth and final approach.
Pfizer added that it will not make a hostile offer direct to AstraZeneca shareholders, instead urging them to press the company's board to begin substantive engagement over a deal.
Pfizer said an offer made to Astra on Friday valuing the firm at £53.50 a share was rejected on the grounds it substantially undervalued the company.
The US company added: "During discussions earlier today, AstraZeneca made clear that it is not currently prepared to accept a price close to Pfizer's £53.50 proposal."
Pfizer chief executive Ian Read said: "We have tried repeatedly to engage in a constructive process with AstraZeneca to explore a combination of our two companies.
"Following a conversation with AstraZeneca earlier today, we do not believe that the AstraZeneca board is currently prepared to recommend a deal at a reasonable price.
"We remain ready to engage in a meaningful dialogue but time for constructive engagement is running out.
"We have said from the beginning that we will remain disciplined in the price we are willing to pay and we will not depart from that guiding principle.
"We believe that our proposal represents compelling and full value for AstraZeneca and that other issues that have been raised by AstraZeneca do not represent material difficulties."
Without further discussions or an extension of the deadline for making a firm offer, Pfizer's proposal will expire at 5pm on May 26.
The company has also increased the cash element of its offer from 33% to 45% and said its latest proposal was 34% higher than Astra's all-time high closing share price prior to the bid speculation.
Viagra maker Pfizer wants to create a new pharmaceuticals giant that will be domiciled for tax purposes in the UK.
It has already pledged in a letter to Prime Minister David Cameron that any deal would not stop the building of Astra's planned research and development (R&D) hub in Cambridge and that 20% of the combined company's R&D workforce will be in the UK.
But Astra believes that Pfizer's board is making a opportunistic attempt to acquire "a transformed AstraZeneca, without reflecting the value of its exciting pipeline''.
Chief executive Pascal Soriot said the mega-merger would prove a distraction from its scientific priorities.
And Pfizer's pledges have also been criticised by the President of the Royal Society, who said they were "vague, come with caveats and have an inappropriate timescale''.