Niki Lauda has called on fellow Mercedes boss Toto Wolff to fully investigate what he believes was an "unacceptable" error in pit strategy at the Monaco Grand Prix.

Despite dominating for the majority of the race, Lewis Hamilton lost out on victory due to an unnecessary pit stop under a late safety car. The tyre change dropped him behind Nico Rosberg and Sebastian Vettel on track and Lauda said the mistake needed to be fully investigated by the team.

"Honestly, I really feel sorry," Lauda said. "I apologised, even to his engineers over there, because this is for me unacceptable.

"Nevertheless, Toto, I gave him the order to analyse it properly and see where the mistake was made. But absolutely the decision to bring him in was completely wrong."

Lauda said there was confusion between the pit wall and Hamilton just before he pitted.

"I was listening to the talks - the talks were going forward and backwards - and there was too much confusion. In the end it was the wrong decision, no question about it."