Prince Charles was secretly questioned by the U.K. police in 2005 over allegations he plotted to kill his ex-wife, Princess Diana.

On Monday, John Stevens, the former head of Scotland Yard, told the U.K.’s DailyMail he had to "follow the evidence" and question the Prince of Wales over a note Diana wrote claiming he was planning a car accident.

According to the outlet, the probe was part of Operation Paget, also known as an investigation into various conspiracy theories – more than 100 of them - surrounding Diana’s death, which was launched by the British Metropolitan Police in 2004.

Stevens told the outlet he spoke to Charles about a note that Diana wrote in 1995 which read, "My husband is planning an accident in my car, brake failure and serious head injury," so that he could marry Tiggy Legg-Bourke, a nanny for their sons Prince William and Prince Harry.

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Princess Diana and Prince Charles divorced in 1996.

Princess Diana and Prince Charles divorced in 1996. (Photo by Tim Graham Photo Library via Getty Images)

The couple divorced a year later in 1996.

The outlet reported that Charles, now 72, was interviewed at St. James’s Palace as a witness two years into the investigation after the note became public in 2003.

"Yes, allegations had been made about the Prince of Wales and other royals but we had to find or examine the [existing] evidence before we approached him with formal questions," said Stevens. "We found no other evidence to support the scenario suggested in Diana’s note."

"We were left with the note, which in itself was not enough to make Charles a formal suspect," Stevens continued. "If he chose to assist [Operation] Paget, he would be doing so voluntarily as a potential witness. We would not be interviewing him under caution."

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Prince Charles with his sons Prince William and Prince Harry and their nanny Tiggy Legge-Bourke at Zurich Airport. The family was embarking on a skiing holiday.

Prince Charles with his sons Prince William and Prince Harry and their nanny Tiggy Legge-Bourke at Zurich Airport. The family was embarking on a skiing holiday. (Photo by Tim Graham Photo Library via Getty Images)

According to the outlet, Stevens read the note to Charles and then asked him: "Why do you think the princess wrote this note, sir?"

"I did not know anything about [the note] until it was published in the media," Charles responded.

"You didn’t discuss this note with her, sir?" Stevens asked.

"No, I did not know it existed," the prince replied.

"Do you know why the princess had these feelings, sir?" Stevens asked.

"No, I don’t," Charles insisted.

Stevens said Charles fully cooperated with the investigation.

"At the end of the day he was incredibly cooperative because he had nothing to hide," said Stevens.

Charles’s father, Prince Philip, declined to assist in the investigation. When asked to comment on the allegations, the Duke of Edinburgh sent a simple message that read: "No, thank you."

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Prince Philip declined to participate in Operation Paget concerning the conspiracy theories surrounding Princess Diana's death.

Prince Philip declined to participate in Operation Paget concerning the conspiracy theories surrounding Princess Diana's death. (Photo by Princess Diana Archive/Getty Images)

The note in question was written around the time that Diana was interviewed by Martin Bashir for BBC’s Panorama. Last month, former supreme court judge John Dyson found that the reporter used "deceitful methods" to secure the televised tell-all.

Bashir, now 58, breached editorial guidelines by creating two false bank statements that he showed to Diana’s brother, Earl Charles Spencer. The 57-year-old then introduced Bashir to Diana and the interview took place two months after the meeting.

In November 2020, Earl Spencer told People magazine the documents were influential in his decision to approach Diana about the interview, as they alleged that a member of his staff was being paid by tabloids to leak information about the princess’s family.

"This was what led me to talk to Diana about such things," he told the outlet. "This, in turn, led to the meeting where I introduced Diana to Bashir, on September 19, 1995. This then led to the interview."

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Martin Bashir interviews Princess Diana in Kensington Palace for the television program Panorama. 

Martin Bashir interviews Princess Diana in Kensington Palace for the television program Panorama.  (Photo by © Pool Photograph/Corbis/Corbis via Getty Images)

Stevens said he regretted that he and his officers didn’t interview Bashir.

"If there’d been an allegation then that Bashir had produced allegedly fake documents to Princess Diana, which is a criminal offense, we’d have investigated it," he admitted. "My goodness me, we would have done [it]. But this has only come out recently, which is unfortunate."

In the infamous interview, Diana said "there were three of us in this marriage," referring to Charles’s relationship with Camilla Parker-Bowles.

Diana died in a Paris car crash in 1997 as she was being pursued by the paparazzi. She was 36.

The Associated Press contributed to this report.