In a single trading session on Monday, about $250bn (£190bn) vanished from the market value of Nvidia. Its share price fell by more than 4% after reports that American officials believed the company's chief executive, Jensen Huang, had been the target of a Chinese espionage campaign. The allegations, first published by the New York Times, centre on Alice Huang, also known as Huang Le, who is unrelated to the Nvidia boss. US intelligence officials reportedly believe she worked for Beijing.

She was managing director of Megaspeed, a Singapore-based company that has had dealings with Nvidia. According to the newspaper, Secretary of State Marco Rubio circulated a memo to senior officials last spring warning that Mr Huang was in a close relationship with her. On Tuesday, President Donald Trump offered the first public hint of what Mr Huang knew. He said he had asked him directly whether he realised she was a spy.

"He said I had no idea she was a spy and I really believe him," Mr Trump told reporters, adding that the FBI was looking into the matter. He gave no detail on when the inquiry began, and the FBI declined to comment. Beijing dismissed the claims as groundless and designed to smear China. Nvidia has commissioned an independent investigation and says it has no reason to believe its secrets or national security were compromised.