The prime minister's mother, Charlotte Johnson Wahl, has died aged 79.
A notice published in the Times newspaper said she died "suddenly and peacefully" at St Mary's Hospital in west London.
Boris Johnson once described his mother, who was a painter, as the "supreme authority" in the family.
Senior figures from across the political spectrum, including Labour leader Sir Keir Starmer, have sent their condolences to Mr Johnson.
Sir Keir tweeted: "I'm very sorry to learn of the prime minister's loss. My condolences to him and his family."