The Princess Royal turns the milestone age on August 15.
And, as per the Sebastian Shakespeare column in the Daily Mail newspaper, the 94-year-old monarch met her daughter for a special socially distanced feast with just the two of them and a small number of the Queen's staff from her ''bubble'' at Buckingham Palace present at the residence on her Berkshire estate.
A source claimed it's ''rare but not unheard of'' for members of the British Royal Family to use the usually empty building for ''special occasions''.
Frogmore is also a more private location for the pair to meet.
The newspaper also claimed that Her Majesty and her husband Prince Philip, 99, will fly to Scotland today (04.08.20) for their traditional summer holiday at Balmoral.
Princess Anne and the Queen's secret lunch came just weeks after the latter attended Princess Beatrice and Edoardo Mapelli Mozzi's wedding at Windsor Castle on July 17.
Meanwhile, a new documentary, 'Anne: The Princess Royal at 70', recently aired honouring Anne ahead of her birthday.
And it saw the Queen learn how to video call with the help of her daughter.
The monarch has found herself needing to brush up on the latest technology in order to communicate with her loved ones and attend socially distanced royal engagements during the coronavirus pandemic.
And Anne taught her the basics of video calling.
The Queen was shown joining a Webex conference to mark Carers Week earlier this year.
She told those on the call: ''Good morning. I'm very glad to have been able to join you.''
Anne - who has children Peter Phillips, 42, and Zara Tindall, 39 - could then be heard asking her mother: ''Can you see everybody? You should have six people on your screen.''
The monarch explained to her daughter she could only see four people, but Anne was quick to quip she didn't actually need to see everyone.
She joked with a laugh: ''You don't need me. You know what I look like.''
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