The 56-year-old was involved in a road traffic accident in Kent in the early hours of Friday morning.
A statement from the Kent Police said that investigations were underway as to the cause of the incident.
PC Glen Braidwood of Kent Police's Serious Collision Investigation Unit said: "A silver VW caddy van, driven by a 56 year-old man from the Essex area, had stopped on the hard shoulder of the M20 Londonbound carriageway at Ashford.
"For reasons yet to be established, the man appears to have entered the carriageway and was then in collision with an articulated goods vehicle. He was pronounced deceased at the scene."
Stepney enjoyed a lengthy career in F1, working for Shadow, Lotus, Benetton and Ferrari.
Having left F1 in the wake of the 2007 spy affair between McLaren and Ferrari, Stepney most recently worked in sportscars - joining the JRM outfit in 2010.
James Rumsey, the owner of JRM, was one of the first to pay tribute to Stepney.
"Nigel was an intense and fierce competitor and always strived for excellence in our racing," he said.
"We certainly could not have achieved our level of success without his leadership and experience.
"Away from the track, he was a focused, driven and passionate member of the JRM Group, and a loving father to his family.
"The rest of the engineering and race team here at JRM learned an unimaginable amount from Nigel in the four short years he was with us and his death this morning has shocked everyone to the core.
"Today, the motorsport world has lost one of its greatest characters and competitors. He will be sorely missed and we send our sincere condolences to his family and the many friends he leaves behind."
- Death & Funeral
- Transport Accident
- Kent Police