(CNN) Britain's Queen Elizabeth II celebrated her 96th birthday on Thursday, in the same year that marks her 70 years on the throne.
The Queen has traveled to Sandringham, her estate in Norfolk, eastern England, for a private break, Buckingham Palace told CNN.
A portrait released to mark the occasion shows the monarch standing between two fell ponies named Bybeck Nightingale and Bybeck Katie. The photo was taken in the grounds of Windsor Castle in March, according to the royal family Twitter account.
William and Catherine, the Duke and Duchess of Cambridge, sent the Queen a birthday message on social media.
"Wishing Her Majesty The Queen a very happy 96th birthday today!" reads the tweet. "An inspiration to so many across the UK, the Commonwealth and the world, it's particularly special to be celebrating in this #PlatinumJubilee year."
Charles, the Prince of Wales and Camilla, Duchess of Cornwall also sent a birthday message to the Queen.
"Wishing Her Majesty The Queen a particularly special 96th birthday today, as we celebrate her Platinum Jubilee year," the pair posted on Twitter.
The Platinum Jubilee marks seven decades since the Queen ascended to the throne in 1952, making her both the longest-reigning British monarch and the longest-serving female head of state in history.
To celebrate the unprecedented anniversary, a number of events will take place throughout the UK over the year -- culminating in a four-day national bank holiday weekend from Thursday June 2 until Sunday June 5, known as the Jubilee Weekend.
Toymaker Mattel has also announced a Barbie doll in the monarch's likeness to celebrate the jubilee.
The Queen Elizabeth II Barbie sports an ivory gown, blue riband and a tiara. It will go on sale from Thursday.
The Queen recently revealed that she was left "very tired and exhausted" following a recent bout of Covid-19. The monarch tested positive for the virus in February.
However, her grandson Harry, Duke of Sussex said the Queen was "on great form" when he saw her in mid-April.
"It was just so nice to see her," he said in an interview with NBC's "Today" show which aired Wednesday.
Ahead of his grandmother turning 96, Harry commented that "after a certain age you get bored of birthdays."
Talking about the relationship he has with the Queen, Harry described it as "really special," adding: "We talk about things that she can't talk about with anybody else."
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