Sauber team principal Monisha Kaltenborn said the collision between her drivers at the Monaco Grand Prix was "unacceptable".

Both cars retired following the collision at the Rascasse corner after Marcus Ericsson attempted to pass Felipe Nasr for 14th place. Ericsson crashed into the side of Nasr's car and was later issued a three-place grid penalty at the next race for causing the collision.

Prior to the accident Nasr had been told to let Ericsson pass but ignored the team order, saying an engine issue and blue flags were to blame for his slow pace. Kaltenborn said the team stuck by its call from the pit wall and would sit down with the drivers to make sure a repeat would not happen at the coming races.

"It was unacceptable behaviour by both drivers," she said. "Today the work of the whole team ended in a collision. Marcus and Felipe both know how much work is put into every race weekend. They have the responsibility to make it to the end of the race.

"After evaluating the overall situation, it was important to bring the fastest car as far as possible to the front, so that we were able to use any chances. Our decision was based on the data from both cars. After this, we have clarified the situation internally and both drivers are aware of their responsibilities.

"Such an incident will not happen again."

Ericsson explained that he had been given the OK to overtake Nasr and thought his lunge up the inside would not lead to a collision.

"I was catching him at three or four seconds per lap, got stuck behind and called for the team what to do," Ericsson said. "They told me they were going to tell him to swap positions and they said to me for seven or eight laps that they were telling him and that didn't happen so after seven or eight laps I said to the team 'we are losing too much time here'. We had so much pace, at least two seconds a lap more pace in the car that we cannot use so I will have to try and make a move myself.

"I got the green light from my engineer to try and make a move and I tried in that corner to overtake Bottas earlier in the race and I almost got him and it was all fine. I thought it was a good place because I saw [Nasr] was struggling a lot in the last sector. So I went for the move and I thought in the worst case I would stay in position and have to drop back behind but obviously the end result was not the same as with Bottas."

Sauber's team order was issued after Nasr had two very slow laps following the Virtual Safety Car for Max Verstappen's accident at Massanet. Nasr picked up his pace soon after and closed up on Romain Grosjean's Haas in 13th place, saying later on that he did not agree with team's assessment of the situation.

"I was doing my own race. First of all, it's Monaco. There were such tricky conditions for everyone. When it goes wet to dry, there's only one racing line for everyone. It was really difficult to overtake out on track in all conditions out there.

"All I can say is that, today, I had to race with an old spec engine, starting from the pitlane, and my race was going really well. But when the leaders came through to lap me, I just kept getting blue flags for 3 or laps and I lost tyre temperature. Then having some engine problems too and trying to diagnose problems from inside the car.

"There was a misfire, and that doesn't help. Once I got it cleared and got my temperatures back, I caught Grosjean and I was right behind him when Ericsson tried to overtake and we crashed at La Rascasse."