They were expected to be joined by a few friends and the Duchess of Sussex’s mother Doria on the day.
The former actress was apparently making a carrot cake, with some help from their son, for the party.
A source close to the Duke and his wife told DailyMail.com: “Meghan is planning a low-key birthday dinner for Harry. In light of what’s happening in the world - from Afghanistan to Covid - they both decided to keep it simple.”
It comes after the couple used the former Suits star’s 40th birthday last month to announce her new charity project.
Watch: Prince Harry and Meghan make TIME Magazine’s 100 most influential people list
The Duchess of Sussex shared a video on the website of her charitable foundation Archewell on August 4 promoting her ‘40x40’ initiative encouraging people to donate 40 minutes of their time to help someone else or to mentor someone in need.
Speaking at the time, she said: “For my birthday I have asked 40 friends, activists, athletes, artists and world leaders to help kick off a global effort by contributing 40 minutes of mentorship to support women re-entering the workforce.”
The Standard reached out to Harry and Meghan’s representative.
Earlier on Wednesday, the Duke was named one of the most influential people of 2021 and appeared on the cover of Time magazine alongside his wife.
The couple have been living in California with their son and daughter Lilibet after they made the US their home.
They stepped down as working royals last year.