Former British Prime Minister David Cameron said he held mock one-to-one audiences with now-King Charles III during the six years of his premiership to help prepare the royal to succeed his mother, Queen Elizabeth II.

“He knew his mother couldn’t last forever,” Cameron told the “CBS Evening News.” “Although for those of us, you know, born since 1952, we’ve always had the queen. And it feels like a rock of our lives has disappeared.”

Elizabeth died on Thursday, hours after her doctors placed her under “medical supervision.” Her eldest son, Charles, immediately became king.

The British monarch’s role includes holding weekly audiences with the sitting prime minister. Queen Elizabeth served 15 U.K. leaders during her over 70 years on the throne, according to The Guardian.

Cameron said Charles for years “thought deeply” about the role of king he would assume one day.

“He wanted to get every part of it right,” Cameron said.

In his first address as monarch to the United Kingdom and the Commonwealth Friday, the new king pledged to devote his life to public service.

“Queen Elizabeth was a life well-lived ― a promise with destiny kept, and she is mourned most deeply in her passing,” Charles said. “That promise of lifelong service I renew to you all today.”

The U.K. is a constitutional monarchy, which means the Parliament is responsible for drafting and passing legislation, while the monarch, now King Charles III, acts as the head of state.

Cameron said that while the role of the monarchy is “mostly ceremonial,” it still holds importance. The head of state, he noted, acts as a “unifying figure.”

Cameron said the queen was “one of the world’s greatest diplomats.”

“Look at what she did to help bring Britain and Germany together after the war,” Cameron told CBS’ Norah O’ Donnell. “Look at what she did to help with the transition to a non-racial South Africa. Look what she did when I was prime minister. Her visit to the Republic of Ireland and healing so many of the wounds in that relationship was remarkable only she could have done.”

Queen Elizabeth visited the Republic of Ireland in 2011, becoming the first British monarch to visit the country since it gained independence from the U.K.

She did not shy away from addressing the period known as “The Troubles” in Northern Ireland — which remains part of the U.K. — between nationalists, who wanted to be part of the Republic of Ireland, and unionists, which ended with the historic Peace Friday Agreement in 1998, according to The Washington Post.

“To all those who have suffered as a consequence of our troubled past, I extend my sincere thoughts and deep sympathy,” the queen said in a speech, according to the Post. “With the benefit of historical hindsight, we can all see things which we would wish had been done differently or not at all.”

Still, many people in some of the British Empire’s former colonies have expressed conflicting feelings about the queen and what she represented.

Cameron is not the only prime minister sharing fond memories of the queen.

Boris Johnson, who formally resigned from the prime ministerial job last week, recalled his final audience with the queen just days before her death, saying she was very “focused,” despite being visibly ill.

“I just thought how incredible that her sense of duty had kept her going in the way that it had. And, given how ill she obviously was, how amazing that she should be so, so bright and so focused,” Johnson told the BBC.

Tony Blair, who served as U.K. prime minister from 1997 to 2007, said he met with the queen a few months ago, saying “she was in amazing form.”

Blair recalled one of his first encounters with the queen when he was new to the prime minister job.

“When I was appointed prime minister, I remember she said to me, her first words to me: ‘My first prime minister was Winston [Churchill], and that was before you were born.’ So she had this extraordinary grip on history,” Blair told NBC’s “Today” show.