Dr Paul Knapman, the Westminster coroner, recorded a verdict of death by misadventure at the inquest in central London. He said that it was clearly not a natural death, as first believed by French doctors, but he ruled out suicide, saying there was no evidence that Lady Rothermere, 63, had been depressed.
The inquest was told that the viscountess, wife of Lord Rothermere, the chairman of Associated Newspapers, was dependent on sleeping pills.
She had twice the prescribed amount of the sleeping drug flurazepam in her bloodstream when she died on 12 August, as well as five times the normal level of an antihistamine, which she was taking as a sedative.
One of her doctors, Douglas Rossdale, told the hearing: 'I think she felt insecure if she did not take her (sleeping) pills. She had a busy life and a lot of functions and she could not take the withdrawal symptoms.'
Dr Knapman said: 'Viscountess Rothermere accumulated many drugs and was incautious with them.' She had reportedly been feeling unwell the night before she died and had probably taken the tablets in an attempt to have a good night's sleep, he said.
More than 2,000 tablets in 75 containers were found in her family's villa at Cap d'Ail, near Nice.
The inquest was told that Lady Rothermere was preoccupied with her health. She suffered from a variety of ailments and had been seeing at least four doctors in London, as well as others in France and New York.
Lady Rothermere's maid, Balbina Pocas, told the inquest that she had gone to bed at 2am on the day she died. When she failed to appear by late afternoon, Miss Pocas climbed into the room through a balcony window, the door being locked, and found her slumped on the floor.
The alarm was not raised until 5.30pm. Her son, Jonathan Harmsworth, told the inquest: 'It was very usual for her to get up at about 5 or 5.30 in the evening.'
When London's premier socialite died, gossip columnists and social diarists lost a rich source of material. A colourful, extrovert figure who was often portrayed as a scatterbrain, 'Bubbles' Rothermere was rarely far from the centre of attention.
Her eccentricities - she apparently enjoyed midnight fish-and- chip feasts in her Bentley - were legendary, as was her extravagant fashion sense. Voted one of London's 10 most beautiful women in her youth, she continued to favour bows, frills and flounces as her figure expanded.
Once described as the world's best party-giver, she entertained international politicians, businessmen, aristocrats and show- business personalities at lavish dinners, often raising large sums for charity.
Flying between homes in London, Sussex, California, Paris, Jamaica and New York, she pursued an energetic life-style, which included skiing, dancing and walking. It was said she would attend the opening of an envelope.
In 1981, Lady Rothermere and her husband spent an estimated pounds 100,000 on an all-night wedding party for one of their daughters, which included the cost of flying a band from New York to their Sussex mansion.
She moved in the highest circles, counting Princess Margaret and Princess Michael of Kent among her friends. Asked recently how she intended to celebrate the start of the milennium, she said that she planned a 'fabulous and gay' party on the rooftop of a New York skyscraper.
According to one anecdote, she once chastised the newspaper columnist Keith Waterhouse at a Buckingham Palace garden party for failing to show enough sympathy for her servant problems and told him he was sacked. Unfortunately, Mr Waterhouse worked for the Daily Mirror at the time - a newspaper which was not part of her husband's media empire - although he later moved to the Daily Mail. (Photograph omitted)