The husband of Eva Rausing, one of the richest women in Britain, is being questioned about her death, reports said.
Hans Rausing, the heir to the Tetra Pak drinks carton empire, was held over the discovery of his wife's body at their home in Cadogan Place, Chelsea.
Mr Rausing, who stands to inherit the £5.4 billion business enterprise built by his Swedish father, also named Hans, had been arrested on suspicion of possession of drugs before the body was found.
Scotland Yard said the death was being treated as unexplained. A post-mortem examination got under way earlier at Westminster Mortuary.
A Scotland Yard spokesman said: "We believe we know the identity of the deceased but await further confirmation." The 49-year-old man who was arrested is being held at a south London police station.
In 2008 the couple were in trouble with the police over drugs but the prosecution was formally discontinued. Mr Rausing, then 45, was charged with drugs offences after police found crack cocaine, cocaine and heroin during a search of his home. After lengthy discussions between his legal team and prosecutors, he accepted a conditional police caution instead.
Neither Mr Rausing nor his wife Eva, who also faced drugs charges, was present at City of Westminster Magistrates' Court that August to hear that the prosecution had ended. The couple had been arrested in April after Mrs Rausing was caught with drugs as she tried to enter the US Embassy in London.
Court documents revealed Mrs Rausing, then 44, was carrying about 10g of crack cocaine, 2.5g of heroin and 2.35g of diethylpropion, a banned stimulant and appetite suppressant.
A further drugs stash, 220mg of diazepam, used to treat anxiety, was also found in her Renault Clio car. The couple's £5 million Georgian five-storey town house in Cadogan Place was subsequently searched.
Officers found 0.2oz (5.63g) of crack cocaine, 0.1oz (2.9g) of heroin and almost 1.8oz (52g) of cocaine. The conditional cautions, administered by a senior local officer, meant the couple admitted possessing the drugs.