From the longer biography on Wikipedia 
Anna Maria Russell, Duchess of Bedford (3 September 1783 – 3 July 1857) was a lifelong friend of Queen Victoria of the United Kingdom, whom she served as a Lady of the Bedchamber between 1837 and 1841. She was also the originator of the British meal "afternoon tea."
whilst visiting the 5th Duke of Rutland at Belvoir Castle in the mid-1840s. During the 18th century, dinner came to be served later and later in the day until, by the early 19th century, the normal time was between 7:00 and 8:30 p.m. An extra meal called luncheon had been created to fill the midday gap between breakfast and dinner, but as this new meal was very light, the long afternoon with no refreshment at all left people feeling hungry. She found a light meal of tea (usually Darjeeling) and cakes or sandwiches was the perfect balance. The Duchess found taking an afternoon snack to be such a perfect refreshment that she soon began inviting her friends to join her. Afternoon tea quickly became an established and convivial repast in many middle and upper class households.