The body of Ms Rausing was found by police on Tuesday when they were searching her house following the arrest of a 49-year-old man, understood to be her wealthy husband, Tetra Pak heir Hans Kristian Rausing.

Mrs Rausing's friend of 15 years, who did not want to be named, said: "I would definitely say she would have died from an overdose.

"Eva was a troubled soul and he (her husband) was equally troubled. She wasn't openly handing around drugs, but you knew she was on it."

Hans Kristian Rausing, whose Swedish father helped transform Tetra Pak into a hugely successful manufacturer, was originally arrested on suspicion of possession of drugs on Monday.

A subsequent search of the couple's five-storey town house in Cadogan Place led to the discovery of Mrs Rausing's body.

It is thought that her husband was then arrested in connection with the death, and has since been moved to a "medical facility" where treatment could last for a number of days.

Scotland Yard would not confirm the name of the man in custody, but said a 49-year-old man had been arrested in south London.

The friend, living nearby in Belgravia, explained she and Eva Rausing had become good friends because their children attended the same school, Garden House.

"She was a dear friend, it's such a tragedy," she said.

"The last time I saw her was a couple of months ago. She had cut herself off from people for a while, especially since the incident at the US embassy."

American-born Mrs Rausing and her husband had met at a rehab clinic in the US and had been battling publicly with their addiction since 2008.

That year, Mrs Rausing was caught in possession of crack cocaine and heroin as she attended a reception at the US embassy in London. All charges were dropped.

Over the years the couple has given many millions of pounds to charity including Action On Addiction, the charity for which Kate, Duchess of Cambridge, is currently patron.

"Eva tried hard to turn things around and went to about every rehab around," her friend said.

"I heard she may have been dead in the house for a while, but it was rare for him (Hans) to be out of the house for more than a few hours.

"They stayed indoors a lot and mostly lived in a small part of the house.

"We had great times together. She was such a fun, gregarious woman. Drug addiction is an illness and it's such a tragedy."

A post-mortem examination on Tuesday failed to identify a formal cause of death. Further toxicology tests are being carried out on Wednesday.

Officers from the Homicide and Serious Crime division are now investigating the case "and the death continues to be treated at this time as unexplained," police said.

Mrs Rausing's family issued a statement saying they are "deeply saddened by the death of their beloved daughter".

Her parents Tom and Nancy Kemeny said: "Eva was a devoted wife for 20 years and mother of four much-loved and wonderful children.

"During her short lifetime she made a huge philanthropic impact, supporting a large number of charitable causes, not only financially, but using her own personal experiences.

"She bravely fought her health issues for many years. The family is devastated at her death and asks to be given privacy at this difficult time."

Her husband's family said: "Hans and Marit Rausing and their family are deeply shocked and saddened to hear of the tragic death of their daughter-in-law Eva Louise Rausing. They ask that their privacy be respected at this sad time."