Alan McGregor said he saw a mystery bellboy drive her car just minutes before she got into her vehicle before her fatal crash in a Paris tunnel on August 31 1997.
The 68-year-old believes a potential murder plot could have been planned six months before her tragic end.
Mr McGregor worked was tasked with protecting the Saudi royal family at the Ritz hotel in the months leading up to her death.
He said: “The night she died my wife woke up to tell me the news.
“My first response was ‘I’m not surprised’.
“I’d seen so many breaches in her security at that hotel and strange goings-on it was bound to happen sooner or later.
“The security that night was ludicrous.”
Mr McGregor told the Sun that a bellboy in a green uniform drove her car to the hotel’s back entrance, where it was left unattended until driver Henri Paul appeared.
He added: “Why was it driven by a bellboy? It should have been a specially-trained driver or a security agent. He could have been anyone.
“You just don’t do that in security — especially when it’s somebody as high profile as Diana.
“Nobody used that entrance either because it wasn’t secure. Any member of the public could have walked up to it.”
He added: “You could say all the elements that led to her death were just a tragic coincidence but part of me believes it could have been six months in the making. While I was working in Angola earlier that year we had lots of meetings to discuss ways Diana might be killed or injured.”
Driver Henri Paul was said to have been three times over the drink driver limit the night of the crash, yet Mr McGregor is skeptical of the claims.
He added: “Paul wasn’t a drinker at all. I never saw him buying alcohol. He just wasn’t that sort of bloke. It doesn’t make any sense.
“I’d been through that tunnel so many times too and it would have been almost impossible to hit that pillar.”