Princess Diana would have turned 65 this year — but nearly three decades after her tragic death in a Paris tunnel, explosive conspiracy theories about what really happened that night continue to haunt the royal family.

Now, former MI6 officer Richard Tomlinson is once again making shocking claims about the crash that killed the beloved Princess of Wales, alleging British spies may have played a role in the deadly 1997 wreck.

Diana, her companion Dodi Fayed, and their chauffeur Henri Paul were killed after their Mercedes slammed into the 13th pillar of the Pont de l’Alma underpass in Paris on August 31, 1997. Her bodyguard, Trevor Rees-Jones, was the only survivor.

Official investigations concluded the crash was caused by a combination of Paul’s drunk driving and the paparazzi chase that followed the couple from The Ritz hotel.

But Tomlinson has long claimed there was much more to the story.

The former spy has alleged Diana was being watched by MI6 for years before her death — and claimed secret files hidden inside the intelligence agency could shed light on what happened in the final hours of her life.

Tomlinson, now 63, has claimed Paul may have been connected to MI6 and that his mysterious movements before the crash deserve closer scrutiny.

“Despite thorough police inquiries, Henri Paul’s whereabouts for an hour have not been accounted for,” Tomlinson previously said.

“I suspect that he was having a drink with his MI6 handler, as a large sum of money was found on his body later that evening.”

French authorities found the equivalent of roughly $2,600 in francs on Paul’s body after the crash. Tests also showed he had a blood alcohol level of 0.19 percent — about three times the legal limit in France.

Investigators also found antidepressants in his system, including Prozac, as well as Tiapridal, a drug sometimes used to treat alcohol withdrawal.

The former Ritz security manager had reportedly been off-duty earlier that evening before being called back when Diana and Dodi unexpectedly returned to the hotel.

According to reports from the time, Paul left The Ritz between about 7 p.m. and 10 p.m. before returning to drive the couple. Questions have long swirled among conspiracy theorists over where he went during that window.

Tomlinson has also pointed to another disturbing detail: Paul’s carbon monoxide levels.

At Diana’s 2008 inquest, medical experts said Paul had unusually high levels of carbon monoxide in his blood, ranging from 12 to 21 percent. A normal reading is usually far lower.

To conspiracy believers, that strange finding has fueled speculation that Paul may have been impaired in a way that was never fully explained.

Tomlinson has also pushed a chilling theory involving a bright flash of light inside the tunnel moments before the crash.

He claimed he once saw a classified MI6 document from the early 1990s describing a proposed assassination plan against Serbian leader Slobodan Milosevic. According to Tomlinson, one version of the plan involved using a strobe light to blind a driver and cause a staged car crash in a tunnel.

“What later struck me about the deaths of Diana and Dodi was that the claims about how they had died mimicked what was in a so-called ‘Y-file’ document on how to assassinate Milosevic,” he said.

“Henri Paul could have been blinded as he drove through the Paris underpass by a high-powered flashlight.”

Some witnesses at Diana’s inquest, including taxi passenger Brian Anderson, testified that they saw a flash of light in the tunnel shortly before the crash.

Tomlinson has claimed that detail matches methods he believes MI6 had previously considered.

“The Y-file proves this was a technique which, at the time of Diana and Dodi’s deaths, was consistent with MI6 methods,” he said.

Tomlinson joined MI6 in 1991 and worked as a targeting officer in the Balkans and Moscow. He was later fired in 1995 and jailed in Britain in 1997 for breaching the Official Secrets Act after writing about his time inside the spy agency.

He went on to repeat his claims about Diana in his book The Big Breach, where he alleged the princess had been monitored for years.

“We were given a standing instruction to keep her in our sights,” he claimed in a rare 1998 interview.

“I don’t think she had one private phone call in years. We had film and tapes of everything she was doing.”

He even claimed that at one point, marines were sent to watch Diana’s home using infrared camera equipment while hiding outside the property.

His claims have never been proven, and official investigations have rejected murder conspiracy theories surrounding Diana’s death.

Still, for millions of Diana fans, the unanswered questions remain impossible to ignore.

Her death stunned the world, shattered her two young sons, Prince William and Prince Harry, and left a permanent scar on the House of Windsor.

Harry has spoken openly about the agony of losing his mother so young — and the pain of knowing she never lived to meet his children, Prince Archie and Princess Lilibet.

Now, as Diana’s loved ones prepare to mark what would have been her 65th birthday, Tomlinson’s explosive allegations are resurfacing all over again.

Nearly 30 years later, the official story says Diana died in a tragic drunk-driving crash while being chased by photographers.

But the conspiracy theories have never died with her.