Queen Elizabeth, the Queen Mother and her daughter Margaret, Countess of Snowdon both passed away in 2002.
Prince Charles - who is first in line to the British throne - has also paid tribute to the queen and his wife Camila, Duchess of Cornwall's mother.
Alongside one picture of Charles, 73, with Elizabeth and Camila, 74, with her late mother Rosalind Cubitt, a message read on the Clarence House Instagram page read: "On Mothering Sunday, we celebrate all the Mothers in our lives and are thinking of those who cannot be with their Mothers today."
The Queen is celebrating the day at home at Windsor Castle.
Meanwhile, it was just revealed that the royal is now using a luxury golf buggy to help her move around in comfort.
Her Royal Highness - who has been using a walking stick at recent engagements due to mobility issues - reportedly received the £62,000 four-seater cart two weeks ago and has already used it to travel around the grounds of Windsor with her beloved dogs.
A source told The Sun on Sunday newspaper: "The cart arrived in the last fortnight. She and her aides have already used it.
"Last week she took the dogs out in it for a tour around the private grounds.
"Her Majesty is struggling with stiffness in her legs, so the cart is a perfect solution to getting her around. She's been through a lot of physical battles recently with Covid and her back sprain, which would take its toll on anyone — let alone someone who turns 96 next month."
The 43mph electric cart was built by Danish firm Garia as part of a tie-up with Mercedes-Benz and is described as putting "many passenger cars to shame".
It has a range of 50 miles before its battery needs recharging and while she's relaxing on one of the four brown leather seats, the queen can tuck into treats from its fridge, or check for weather updates and mapping on the tablet-style 10in screen next to the steering wheel.
The buggy has "weather enclosure" doors and windows, so the monarch will be able to use it all-year round, and it also has headlights, 14in alloy wheels, a lockableboot, heated windscreen, cup holders and even Bluetooth speakers.
Queen Elizabeth The Queen Mother regularly used a golf buggy to attend events in the years before her death aged 101.