Hans Kristian Rausing, the heir to the Tetra Pak fortune and one of Britain's richest men, is being questioned by police after his wife was found dead in their Chelsea mansion.
Eva Rausing, 48, was found in a bedroom of their Cadogan Place home by police searching the premises for drugs in connection with the earlier arrest of her husband.
A Scotland Yard spokesman said: "On July 9 a 49-year-old man was arrested on suspicion of possession of drugs. Following a subsequent search of an address in Cadogan Place, SW1, a body was found. The death is being treated as unexplained.
"A post-mortem examination was conducted at Westminster mortuary.
"The man was further arrested in connection with the death and is in custody at a south London police station."
The couple faced drugs charges in 2008 when large quantities of heroin, crack and cocaine were found in the house. Mrs Rausing was arrested for attempting to smuggle drugs into the US embassy in London.
The couple are one of the country's wealthiest, with Hans Kristian standing to inherit the £5bn Tetra Pak business and Eva being the daughter of a former Pepsi executive.
Forensics experts are examining the couple's home, which they bought after selling their previous house for £12.65m in 2006. They also own a mansion in Barbados and an apartment on The World, a cruise ship for the mega-rich that sails endlessly around the planet.
Hans Christian Rausing's grandfather Ruben owned the Tetra Pak company which his father, also called Hans, developed into a massive financial success.