Catherine, Duchess of Cambridge tweeted a rare reply to a well-wisher as she celebrated her 40th birthday at the weekend

The video message was shared on social media by Mila's mother Lynda as the child sent her best wishes to the Duchess on her special day.

Mila said: "Happy Birthday Catherine! I'm so happy you're turning 40, hope you get nice gifts and love spending time with your family. Bye, lots of love, Mila."

In a surprise move, a tweet from Catherine and her husband Prince William's official account responded to Mila, saying: "Thank you Mila!"

Mila featured in Catherine's 'Hold Still: A Portrait of Our Nation'

photography project which documented the nation during the coronavirus lockdown.

She was pictured kissing her dad through a window as she underwent intensive chemotherapy for leukaemia and had to shield during the height of the global health crisis as the image left a lasting impression on the Duchess.

The pair met in person at the Palace of Holyroodhouse last year and Catherine wore a pink dress after Mila revealed that it was her favourite colour.

As she walked into the room, Catherine said: "Hi Mila, look at you. I want to give you a big, squeezy cuddle. It's so nice to meet you in person."

The royal kept a promise she had made to Mila when they first spoke on the phone the previous autumn after learning about her passion for pink.

She said at the time: "I'll have to make sure I try and find a pink dress. Hopefully when, one day, hopefully, Mila, we'll get to meet and then I'll wear remember to wear my pink dress for you."