Kate Middleton, the Duchess of Cambridge, turned 39 on Saturday, January 9, although her celebrations are thought to have been kept to a minimum, a royal expert has said.

With the UK in lockdown on Kate's birthday, the duchess will not visit her parents Carole and Michael Middleton.

Vanity Fair reports being told by a friend of Kate's: “Catherine isn’t one for lots of fuss or big parties—being with William and the children is her favourite way of spending her birthday.”

And nevertheless, the Duchess of Cambridge has been inundated with birthday messages.

The Queen's Instagram birthday message, posted on Saturday morning alongside a photo of Kate beaming as she enjoys her role as Joint President of the UK Scouts and one of the Queen and Duchess at The Chelsea Flower Show in 2019, read: "Wishing The Duchess of Cambridge a very happy birthday today!"

Kate will spend her birthday in lockdown

In a post shared on Instagram, Clarence House also shared a sweet photo of the Duchess and wrote: "Happy Birthday to The Duchess of Cambridge!"

And the Cambridges replied to all of the well-wishers, posting a photo of Kate wearing a facemask with the caption: "Thank you for your kind wishes on The Duchess’ birthday.

"Birthdays have been very different in recent months, and our thoughts continue to be with all those working on the front line at this hugely challenging time."

It is reported that Kate will want to 'get straight back to work' following her big day

And the message appears to be very telling of Kate's mindset right now, with one royal expert telling Vanity Fair that she will want to "get straight back to work" following her big day.

“Kate will want to get straight back to work, which won’t be easy with the little ones at home, but she has shown how important her work is to her,” said royal author Ingrid Seward. “It’s not just shaking hands, she’s got to the root of the Early Years campaign and she’s going to be doing much more in this area.”

Royal aides say the couple will be resuming their virtual work, and that while traditional engagements and overseas tours remain on hold, there is plenty of work going on behind the scenes.

“It will be back to mostly virtual events for the next couple of months,” Vanity Fair reports being told by a palace source.

Seward added: “They will be visible, they have to be and there’s a lot to do. They’ll be relied upon to lift the nation’s spirits and Kate and William have proved they are very good at doing that.”