The Indian writer and historian Amaresh Misra is reported to have written a book, In War of Civilisations: India AD 1857, which describes his calculations that British soldiers killed or allowed almost 10 million people to die over 10 years, from 1857.
“It was a holocaust, one where millions disappeared. It was a necessary holocaust in the British view because they thought the only way to win was to destroy entire populations in towns and villages. It was simple and brutal. Indians who stood in their way were killed. But its scale has been kept a secret,” Misra told the Guardian.
I’ve added this summary to the Wikipedia page on genocides:
The Indian Rebellion, also referred to as the Indian Mutiny (controversially so, by British historians) of 1857 was a period of armed uprisings against the colonial British East India Company in India between early 1857 and mid 1858. The British response to the rebellion, which was led by the sepoys of the Bengal regiment, was extremely brutal. Official histories record that 100,000 rebellious native solders were killed by the British in their efforts to quell the rebellion. Until recently however, there has been little serious examination of the numbers of Indian civilians killed at the same time.
An Indian historian, Amaresh Misra, has completed a new book, War of Civilisations: India, AD 1857 (New Delhi: Rupa & Co.), which argues  that, “New research reveals that the 1857 uprising encompassed not only the entire Indian subcontinent but also several castes, communities and classes.”
Misra believes that up to 10 million people may have been either killed in, or died as a result of, British reprisals following the uprising. His evidence, which remains controversial, is based on harrowing eyewitness accounts from British soldiers and civilians, and on labour force records kept by the British, which show a dramatic decline in the period following the uprising.