Merlier won the 182.9-kilometre stage in four hours, one minute and 28 sec in a sprint finish ahead of Alpecin-Fenix teammate and fellow countryman Jasper Philipsen, while Frenchman Nacer Bouhanni of Team Arkéa Samsic completed the podium places.

Alpecin-Fenix rider Mathieu van der Poel of The Netherlands retained the overall lead, eight seconds in front of Frenchman Julian Alaphilippe of Deceuninck-Quick-Step after finishing in seventh position in the stage in Pontivy.

"This is the biggest victory of my career so far - a dream come true," said Merlier.

"After my victory in the Giro d’Italia I was already very happy, and now I’ve won a stage at the biggest race of the world. 

"I just can’t believe it."

Britain’s Thomas of Ineos Grendaiers dislocated his shoulder following a heavy fall before joining the peloton after having it put back in place by medics.

But Thomas, winner in 2018, is now expected to undergo a scan to find out the extent of his injury.

Slovenia’s Roglič, last year’s runner-up, hit the ground with nine kilometres remaining in the race after a touch of the wheel of another rider, while his Dutch Jumbo-Visma team-mates Robert Gesink and Steven Kruisjwijk also suffered blows following crashes.

Roglič, who went from fourth to 20th in the general classification, has vowed to continue in the race after an x-ray revealed he had not sustained any broken bones.

"Luckily we saw that everything is still in one piece, nothing is broke, I’m all open all around but I’d say not the best day for us, but we got on and we can continue," Roglič said.

"It was a super stressful final with these roads. 

"Not the best day for us but we continue."

The race is due to continue tomorrow with the 150.4km fourth stage from Redon to Fougères.