Takeover speculation surrounding UK-based drugs giant AstraZeneca pushed up its stock market valuation by more than £4 billion today.
The group declined to comment on weekend reports that larger US rival Pfizer had made a tentative approach that could possibly value it at more than £60 billion.
Analysts expressed scepticism about the possible deal with experts at Deutsche Bank saying it would require a "leap of faith" in AstraZeneca's new drugs pipeline.
But shares were almost 10% higher at one point, adding £4.6 billion to its worth, though this later settled back to around 7%.
Senior investment bankers and industry sources told the Sunday Times that informal conversations about a possible tie-up had taken place in recent weeks.
Astra was reportedly resisting the advances, with no talks currently under way, but it was said that Viagra maker Pfizer could come back with a fresh approach - a move that would mark the biggest-ever foreign takeover of a UK business.
AstraZeneca, which employs more than 50,000 people around the world, will be under pressure to shed light on any discussions when it publishes first quarter financial results on Thursday.
The company was itself formed through the merger of Sweden's Astra and Britain's Zeneca in 1999. It is one of the world's biggest pharmaceutical companies and produces a large range of medicines including cancer and diabetes drugs.
Revenues were down 6% to $25.7 billion last year, reflecting the loss of exclusivity on a number of drugs it had developed as patents ran out.
Pfizer's revenues of $51.6 billion for 2013 were also down 6% on the previous year. A takeover of AstraZeneca would dwarf its previous deal to buy rival Wyeth for $68 billion in 2009.