A tragic young couple found dead in a car were killed after breathing in poisonous fumes from the air vents caused by a vehicle modification went wrong, a UK inquest has found.

Nikki Willis, 23, was found in a blue Ford Fiesta in Chelmsford alongside her car enthusiast boyfriend Tom Putt, 20, in December last year, The Sun^[1] reports.

Cops initially cordoned off the area suspecting a "chemical incident" after a potentially noxious substance was detected — but later declared it safe.

The car was found outside the home Ms Willis shared with her mum Michelle Hindson, 46, and stepfather Stuart Hindson, 45, back in December.

On Tuesday, coroner Caroline Beasley-Murray described their deaths as a "tragic, tragic accident".

At the inquest police warned of the dangers into car modification, the BBC^[2] reports.

The coroner heard how during tests smoke from the car's engine came up through the bonnet and into the air vents before passing into the car.

Chelmsford Coroner's Court heard that the Ford Fiesta had had its catalytic converter removed. A video is said to have shown that vents had also been cut into the car's bonnet.

The Daily Gazette^[3] reports Detective Inspector Robert Kirby, who forensically investigated the car, told the inquest: "A unique series of events came together to allow this tragedy to happen."

A video of Mr Putt's car backfiring flames was posted to social media just two weeks before their deaths. The car enthusiast regularly posted pictures and videos of his car online, including one showing sparks coming from the exhaust.

Following the tragedy, friends and family paid tribute to the pair and a tribute video showed photographs of them and Mr Putt's cars set to music.

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