The first black man to test a Formula 1 car has called triple world champion Sir Jackie Stewart "racist".

It comes after reigning world champion Lewis Hamilton said Stewart's recent comments about racism in Formula 1 - that there is not "as big a problem as there might seem" - were "disappointing".

"Sir Stud Field Mouse aka Jack Stewart was by far the most racist and hostile driver I ever dealt with in my career," Willy T Ribbs alleged on Twitter.

Ribbs, now 65, became the first black man to drive a Formula 1 car in 1986 following an invitation from then Brabham team owner Bernie Ecclestone.

Ribbs said a month ago that he has "respect" for Ecclestone, even though the former F1 supremo had claimed that "in many cases black people are more racist than white people".

As for 81-year-old Stewart, Ribbs told him via social media: "The reason you don't see a diversity issue is because you are the diversity issue."