Inside Kate Middleton's 'Special Birthday Surprise' For Husband Prince William: Report

Kate Middleton prepared a birthday surprise for her husband, Prince William.

The Duke of Cambridge turned 40 years old Tuesday, and the Duchess of Cambridge pulled out all the stops for her husband's milestone birthday.

"Kate is organizing a special birthday surprise for William – an intimate party with friends," an anonymous source told Us Weekly.

The bash Middleton was organizing wasn't an "over-the-top" event that cost "a fortune," with the future king and queen consort focusing on being "low-key," according to the insider.

The duchess' plans for her husband's birthday reportedly also included a "trip to the coast" together.

Prince William has "no big hang-ups about turning 40" because the second in line to the British throne "looks at every passing year as an achievement" in his life, the insider said.

Royal correspondent and commentator Neil Sean also shared some details about the royal's 40th birthday in an article published on Fox News.

According to Sean, having Middleton in his life helped ease Prince William into reaching his recent milestone.

"It's always [a] sense that it's a dread, but honestly, we should all rejoice in these and any birthdays," Prince William told Sean of reaching 40, according to the royal expert.

Sean confirmed that Prince William's birthday celebration would be "private." His and Middleton's three children — Prince George, 8, Princess Charlotte, 7, and Prince Louis, 4 — helped their mom bake their dad's "favorite" cake and made handmade cards and gifts for their father, according to the royal correspondent.

"[Prince William] plans to spend the day quietly but [will] still check in on his charities that he really does care about," Sean continued.

The Duke of Cambridge also asked royal fans not to send cards, flowers and gifts and instead to donate to the less fortunate, such as supporting British magazine The Big Issue, which is known for supporting the homeless and those struggling financially.

Russell Myers, the royal editor of the Daily Mirror, told "Today Extra" that Queen Elizabeth agreed to throw a big party for Prince William and Middleton. International Business Times could not independently verify this information.

According to Myers, Middleton, who also turned 40 on Jan. 9, could not celebrate her birthday earlier this year because of the surge in COVID-19 cases in the U.K. at the time. The joint party will be held at Windsor Castle later this summer, the journalist added.