Princess Kate and Sir Paul McCartney’s inside joke during re-opening of the National Portrait Gallery

Kate was in a fit of giggles after meeting the former Beatles singer and his wife Nancy. Photo / Getty Images

Princess Kate and Sir Paul McCartney appeared to have a lot to giggle about during a recent interaction at the reopening of the National Portrait Gallery.

The Daily Mail has reported the mum-of-three, 41, the Beatles icon, 81, and his wife, Nancy Shevell, 63, shared a joke about aging when they met at the reopening of London’s National Portrait Gallery on Tuesday after its three-year, £40 million ($82.6 million) renovation.

Touring the building, Kate reportedly laughed when ‘Macca’ – who has his own photographic exhibition opening there next week featuring images he personally took at the start of Beatlemania between 1963 and 1964 – joked about whether she or McCartney’s wife would have even been born when the photos were taken.

Kate noted she wouldn’t have been, but McCartney’s wife reassured him she had.

The princess told the Hey Jude singer “Hello, nice to meet you too,” before he told her the staff at the National Portrait Gallery had been “amazing” and “great”.

Kate said: “It’s amazing how long the planning is, how many conversations happen pre-everything.”

Catherine, Princess of Wales leaves the gallery during the reopening of the National Portrait Gallery. Photo / Getty Images

The former Beatles star continued to say the contents of his exhibition were done “when you weren’t even born”, before he told his wife: “Oh, you weren’t either.”

Shevell then reportedly looked up and counted before she replied that she very much had been resulting in the trio erupting into giggles.

It comes after the princess got candid about her early days as a royal and the struggles she faced at the time during a visit to a youth charity last month.

Speaking to a group of girls from a nearby school in Bristol, she opened up about her experience joining the monarchy, revealing to the girls that the thought of becoming a royal never crossed her mind until she fell in love with Prince William, according to one of the people present.

Dame Kelly Holmes, who was also present at the event, told the Daily Mail: “They were just asking her about being a royal. Was it something that she wanted to do? And she said she had to learn. It’s a struggle to know that you can be accepted and fit in and you are still learning every day.

“Doing public speaking isn’t a natural thing for lots of people is it? And she was saying she is still working that out but has to project. So you know again, she humanised everything. Not everyone is perfect.”