Pfizer has raised its offer for its British rival AstraZeneca to more than $US116 billion.
The American pharmaceutical company says it will walk away if the offer is not accepted.
The new offer of £55 per share would value AstraZeneca at about £69 billion.
The offer constitutes a 15% increase from Pfizer's last proposal on 2 May.
Pfizer wants to create the world's largest drug company, with its headquarters in New York, but based in Britain for tax purposes. Corporate tax rates are lower there than in the United States.
However, previous bids have met with opposition on both sides over the Atlantic over fears that research and jobs will go.
The BBC reports the government has powers to veto certain deals, such as those involving defence and media companies, and apply a "public interest test".
But this is rare, and the government's scope to intervene is also limited by the European Commission.