“Mystery” surrounds the death of a mother from Kings Langley, after a coroner ruled she died accidentally following a motorway crash.

Eyewitnesses told an inquest they saw flashes of “orangey-yellowy fire” inside the vehicle of Debra Collett, 44, of Coniston Road, before her black BMW 325 coupé veered “severely“ onto the hard shoulder of the M25.

Amersham Coroners Court heard the car then hit a concrete barrier and spun through the air, before Mrs Collett was thrown from the vehicle and it became engulfed in flames.

However, as collision investigators could not determine what caused the car to swerve from the road so suddenly, her death was ruled as an accident.

Giving evidence at the inquest on Wednesday, witness Glenn Horobin said he joined the northbound M25 carriageway at around 6.40pm on April 14 this year.

As he travelled between junction 16 (M40) and 17 (Maple Cross), he noticed a black BMW that “for no apparent reason veered severely from the middle lane to the hard shoulder” at around 7pm.

He said: “I clearly saw a bright flash appear from inside the car.”

Mr Horobin told the court that after seeing the car hit the crash barrier, it “went airborne and flipped over. I could see fire on the underside of the vehicle. The BMW was now well ablaze and appeared to be a fireball”.

Another witness, Alan Campbell, told the inquest he was driving his Ford Focus along the same stretch of carriageway when he noticed a car overtaking him on the hard shoulder.

He said: “As it was knocking against the barrier I could see orangey-yellowy fire coming from the car. There was an explosion and it was in the air.”

Mr Campbell said that when the car came to a stop he saw a little girl, Mrs Collett's youngest daughter, climb out of the vehicle, which was “blazing furiously”.

He added: “I saw flames underneath the vehicle as it scraped along the barrier but as it flew through the air, I saw flames inside the vehicle.”

The court was also read statements from two further witnesses.

Mahdi Mousabi, who runs a restaurant in Radlett, said he saw a little girl screaming as a stream of passing motorists rushed to the aid of Mrs Collett.

Barbara Cullen added: “I could see flames and smoke around the front of the car. After the little girl was out of the car, the flames took over completely.”

PC Adrian White, from the Collision Investigation Unit, told the inquest the car was found in a field “extensively damaged by fire”.

He said: “Once the vehicle had ignited in the field, it sat in its fluids of petrol and oil and brake fluids and engulfed itself in fire.”

Asked questions about whether a fire inside the car caused Mrs Collett to swerve off the road, PC White said: “There's no physical evidence to say there was a fire, but there's no evidence to say there wasn't.

“It's highly unlikely but I cannot discount it completely.”

Dr Peter Johnson, a pathologist, revealed the catalogue of burns, bone fractures and bruisings Mrs Collett suffered in the incident, and listed her cause of death as “extensive multiple trauma injuries”.

Recording a verdict of accidental death, coroner Richard Hulett said: “Unfortunately we are left with some strange mysteries that does happen in these cases.

“For reasons which are largely obscure the vehicle takes off to the near side. The vehicle goes off rapidly and suddenly towards the near side onto the hard shoulder so it's now appearing to pass people on the near side.

“The progress of the vehicle goes along the grass, finds its way behind a stretch of barrier and then begins to collide with the uprights of the barrier and then there's a major collision.

“We know the driver was severely burnt. We know Mrs Collett sustains some serious burn injuries but her death is due to the trauma of the incident.

“Why the car went behind the barrier remains a mystery. There's no suggestion of any other emergency other than the vehicle travelled to where it did. I cannot believe that an experienced driver would have taken that course of action without some crisis initiating it.”

He added: “Accidental doesn't mean blame free. It means something entirely unwanted and unintended happened.”