Kate Middleton’s ‘private’ birthday with George, Charlotte and Louis as Prince William attends London Air Ambulance anniversary

IT might be the Duchess of Cambridge's 37th birthday today, but we've heard the royal is having a chilled, low-key day as Prince William attends an important engagement.

While Kate's plans haven't been confirmed, it's expected she's been having a rather relaxed birthday this year with the couple's three children, Prince George, Princess Charlotte and Prince Louis.

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Much of her day has been spent without Wills as the Duke attended a tea party celebrating 30 years of the London Air Ambulance.

It's been a special day for Will, too, after he was announced as the Patron of the Air Ambulance's campaign, allowing him to "champion those working on the frontline, and highlight the importance of the London Air Ambulance’s vital work".

Prince William, 36, also met clinicians, paramedics, pilots and crewmembers to discuss the challenges the teams face while responding to emergencies in the capital.

London’s Air Ambulance has been providing crucial and life-saving aid to Londoners for the past three decades and, with Will having worked as an air ambulance pilot himself, the Duke knows first-hand how vital the charity is.

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The birthday girl, meanwhile, is said to have spent the day at Kensington Palace.

And with the children having returned to school and nursery yesterday, it's believed Kate will have been doing the school run too.

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But that's not to say Kate hasn't celebrated her 37th.

We're sure she'll have had an adorable celebration with George, Charlotte and Louis - and hopefully they'll have saved a slice of birthday cake for Wills, too.

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Earlier this week, we told you Meghan Markle and Prince Harry’s first official engagement of the year will be a visit to Merseyside.

We also revealed Meghan Markle’s baby will be Christened in a lavish £11,000 font that’s used for all royal newborns.

And we showed you how Kensington Palace shared a "beautiful" photo of Kate to mark the Duchess' birthday.