During the 19th century, it was the custom to take an early breakfast and a late supper. Anna Maria Stanhope, the seventh Duchess of Bedford (1783-1857), found that this long gap between meals resulted in a "sinking feeling" at about four o'clock, and so invented afternoon tea as a way to fill the gap.
At first, the Duchess had her servants sneak her a pot of tea (likely English Breakfast, Earl Grey, Lapsang Souchong, or Darjeeling) and a few breadstuffs.
Later adopting the European tea tradition, she was fond of inviting friends for tea in her rooms at Belvoir Castle. Thus, she created a fashionable practice among ladies of the upper classes - small cakes, bread and butter sandwiches, petit fours, and delectable teas.
The Duchess continued this tea practice upon her return to London, sending a stream of invite cards to her friends asking them to join her for "tea and a walk in the fields."
Its popularity spread, and four o'clock tea became a daily ritual of British life, and for some, the main meal of the day.