Kate Middleton, pictured with her mum Carole, has two very big reasons to celebrate on June 21 (Image: WireImage)

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Kate Middleton has a double reason to celebrate today as it isn't only her husband's 39th birthday but also her parents' wedding anniversary.

While it is well-known that Prince William's birthday is on June 21, less people are aware that the Duchess of Cambridge's parents Carole and Michael Middleton also celebrate their anniversary on such date.

Carole and Michael, who live in Berkshire, are today celebrating 41 years of being together since they tied the knot in 1980.

When Carole and Michael met, she was a British Airways air hostess and he a flight-dispatcher for the same airline.

They married in the quiet village of Dorney, Buckinghamshire, almost exactly a year before Prince Charles wed Diana Spencer and kissed on the Buckingham Palace balcony.

Carole and Michael Middleton celebrate their 41st wedding anniversary today
(Image: Steve Reigate)

The Middletons, who also have a younger daughter, Pippa, could never have imagined back then that 30 years on they would be standing on the same balcony looking out as their own daughter kissed her prince.

While the Middletons celebrate their special day together, Prince William is receiving birthday wishes from all over the world as the second-in-line to the throne hits 39.

And his wife Kate, 39 and who he met while at the University of St Andrews in 2001, is likely to "pull out all the stops" to make his big day extra special, according royal expert Katie Nicholl.

Nicholl said she thought mum-of-three Kate would do everything she could to make sure her husband enjoys his special day after enduring a difficult period due to his rift with Prince Harry, 36.

Prince William, pictured with George and Charlotte, celebrates his 39th birthday on June 21
(Image: Ian Burt)

This birthday will also be the first without his granddad Prince Philip, who died aged 99 on April 9.

The author predicts Kate and William will spend the day at Kensington Palace with their children Prince George, seven, Princess Charlotte, six, and Prince Louis, three.

Ms Nicholl told OK Magazine: "Kate always makes birthdays and celebrations special for William and the children.

"She's very much like her mother Carole in that respect. She'll be aware that this year has been tough for William and will want both his birthday and Father's Day to be special for him."

According to Ms Nicholl, the Duchess of Cambridge will bake a cake to celebrate her husband's birthday and their children will make homemade cards and gifts.

The Queen on Monday led the well-wishes for William with a tweet where she said: "Wishing The Duke of Cambridge a very Happy Birthday today! #HappyBirthdayHRH."

The Queen also shared four pictures of her grandchild either on his own or next to his wife and children.