Both Princess Anne and Prince Philip had key senior royal roles for many years.
Prince Philip officially retired from royal duties in 2017 and has not been seen out very often since then.
However, pictures with Princess Anne over the years show their close relationship.
Body language expert Judi James explained how their similarities may have helped them stay close.
“Historically, Anne has always been known as being closest to Prince Philip,” Judi told Express.co.uk.
“Like her father, Anne appears to be someone stoic and dependable but also unfussy and upbeat as a companion.”
She revealed the duo may have taken a similar approach to how they worked as part of the Royal Family.
Judi added: “She is very much the chip off the old block as far as Philip is concerned.
“Like her father, Anne appears to make the Queen laugh,” the expert continued.
“Most poses between the two women suggest a shared sense of humour, bringing out a side of the Queen that we rarely get to glimpse.
“Like Philip, Anne often looks on the brink of a wry and slightly cynical smile.
“This more down-to-earth and even irreverent approach should be exactly what the Queen needs to keep her happy and grounded.”
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The royal line of succession – who outranks who?
The line of succession to the British throne dictates the order in which each member of the Royal Family would ascend to the throne.
It is also seen as a ranking of importance with the head of the line, the Queen, taking the place of ruler.
Older children come before younger children. Traditionally boys came before girls, but this law was changed on 26 March 2015 before the birth of Prince William’s first child.
Incredibly, Catholics are still excluded from the line of succession, as are children born outside of wedlock.
The royals, who usually stick to a strict protocol when appearing in public, often arrive at events in ascending order of importance, with the most important royal arriving last.
Prince Charles, 71, is currently first-in-line to the British throne, followed by Prince William, 38, his oldest son.
Then comes Prince William’s children, George, seven, Charlotte, five, and Louis, two, and they are followed by Prince Harry, 36. Prince Harry is succeeded by his son Archie Harrison, born in May 2019.