The M-Sport Ford driver misjudged a left hander resulting in his Puma ploughing into a rock face, triggering a series of rolls over the Armco barriers, with the car coming to rest 30 metres into a ravine.
Luckily Fourmaux and co-driver Alexandre Coria emerged unscathed from the crash but the Puma suffered significant damage.
Fourmaux had been sitting fourth in the overall standings, 17.9s adrift of rally leader Toyota's Sebastien Ogier after an impressive start to the rally last night.
M-Sport team principal Richard Millener confirmed that the car is heavily damaged, before admitting Fourmaux's rally is most likely over with a repair unlikely.
"He has had a very heavy accident by the looks of things," said Millener.
"From what I can see it looks like quite a dirty corner he has gone to cut the line and I guess the cars in front have pulled a bit of mud out, and he just went head on into a mountain, and it has just flipped him over the barrier and 20 to 30 meters down the bank.
"The car looks fairly heavily damaged so I suspect that will be the end of the rally for this one which is a real shame.
"It is not what we wanted as we talked about making sure we get to the end, and it is difficult when they feel like they have good confidence and his comments last night about the car last night were very positive and he took that into today.
"It is another hard lesson in the development cycle, there is not much we can do."
It proved to be bittersweet start to Friday for M-Sport, as its fourth entry piloted by nine-time world champion Sebastien Loeb won the third stage to cut Ogier's rally lead to 5.5s.
The stage win is Loeb's first of the rally after witnessing his old rival Ogier win the first two stages last night as the Monte Carlo specialists went head-to-head.
