Shares in Britain's second-most valuable company slumped on Monday after it emerged that AstraZeneca had held talks about a potential $400bn (£300bn) tie-up with Bristol Myers Squibb. The potential deal would also raise fears that AstraZeneca could shift its business to the US in a blow to Britain. AstraZeneca is worth around £184bn on the London Stock Exchange, while New York-listed Bristol Myers Squibb has a market value of $133bn. Bristol Myers Squibb shares surged as much as 3.8pc on Wall Street on Monday. The potential deal could also trigger a political backlash from Donald Trump's administration, Jefferies warned.