The broadcaster said the high numbers accounted for “the majority of the UK public”.
Meanwhile, live footage of the Queen lying-in-state was streamed 25 million times across BBC online, it said.
“Yesterday was BBC One’s biggest viewing day since the 2012 Olympics Closing Ceremony,” a spokesperson said.
“At peak, 22.4 million people were watching across all BBC channels.”
The broadcaster added that the viewing figures it provided did not include live streaming on mobile devices or those watching on catch-up, so the final consolidated figures are likely to be higher.
The BBC’s announcement comes shortly after the new culture secretary, Michelle Donelan, suggested that the state funeral was taxpayer money “well spent” amid the cost of living crisis.
Tuesday (20 September) marked the start of a period of royal mourning for Britain’s longest-reigning monarch and saw King Charles II head to Balmoral Castle in Scotland, which is believed to have been his mother’s favourite place.
The royal family are not expected to carry out any official engagements for a further seven days during this time.
This story is being updated