Heidfeld got alongside Prost going in the the left-hander, but Prost decided to shut the door. This sent Heidfeld in a spin.
Out of control, the German's car clipped the curbing and took off, only to land after almost two full barrel rolls.
The former F1 driver was luckily not injured.
"I had kept a lot of energy while still racing hard," Heidfeld is quoted saying by Auto Hebdo magazine.
"I got alongside Nico in the last corner and then I couldn't do anything."
At first, Prost refused to take responsibility for the crash. But upon watching the replays, he changed his mind.
"I feel really bad for causing this accident," he said. "I've realized that I was responsible for all this after watching the videos. I simply did not see him.
"The most important thing is that my friend Nick Heidfeld is alright. I'm really sorry Nick. You know I would never do something like that."
Heifeld, who is Prost's teammate at Rebellion Racing in the World Endurance Championship (WEC), accepted the Frenchman's apologies.