Prince William, Harry’s former royal mentor reveals mother’s secret MI5 past after decades of silence

Jamie Lowther-Pinkerton grew up knowing his mother had led an "amazing life." But there was one chapter she rarely spoke about.

After World War II, Sue Lowther-Pinkerton joined MI5 and worked in British intelligence during the Cold War.

But she revealed little to her children about that part of her past.

Decades ago, while watching a documentary with her children, Sue let something slip.

KATE MIDDLETON SCALES UK'S THREE HIGHEST PEAKS IN 24 HOURS WITH A DEEPLY PERSONAL MESSAGE FOR CANCER SURVIVORS "We were watching a documentary one afternoon about 30, 40 years ago," Lowther-Pinkerton recalled to Fox News Digital.

"And there was a thing called the ‘Cambridge spy ring,’ which were Soviet agents who had infiltrated the British system.

And there was a great mystery about who the fifth man was.

And up pops the documentary maker saying, ‘Well, it could be Sir Roger Hollis.’" Hollis served as director general of MI5 from 1956 to 1965.

He later became the subject of allegations that he had secretly worked for the Soviet Union, claims that were investigated but never proven.

Sue didn't hesitate.

WATCH: PRINCE WILLIAM, HARRY’S FORMER ROYAL AIDE REVEALS MOTHER'S SECRET PAST "My mother, without missing a knitting stitch, said, ‘Absolute nonsense.

He was a total gentleman and a patriot.’" "[My sister and I] both looked at her sideways as if she was completely mad," Lowther-Pinkerton said.

"I tried to get more out of her, but she was of that generation who quite rightly just didn’t want to talk about it." Lowther-Pinkerton said he was in his mid-20s before he and his older sister learned of their mother's intelligence career.

MI5, formally known as the Security Service, is Britain's domestic intelligence agency.