Lando Norris has hinted that he was taken by surprise by his McLaren team-mate's decision to try and overtake him during the first lap of the Italian Grand Prix.
In a blow to the team's constructors' and drivers' championship chances, Piastri finished second behind Leclerc with Norris third, not able to maximise Max Verstappen's disastrous sixth-place finish.
McLaren championship hopes damaged
Following a decent start from Norris, the pole sitter managed to keep both Piastri and George Russell behind him heading into Turn 1, before Piastri got the better exit heading towards the second corner.
The young Australian made a move on his team-mate into Turn 2, seemingly taking Norris by surprise as the Brit suffered a wobble which also allowed eventual winner Leclerc to get past him.
Norris' race engineer Will Joseph later told the Brit over team radio that the pair would be allowed to follow 'papaya rules', suggesting that they were allowed to race each other despite fighting for both world championship titles.
Speaking after the race, Norris suggested that Piastri's move may have put their constructors' championship hopes at risk too.
"I feel he got way too close for comfort," he told Sky Sports.
"We could both have easily been out in that first corner if I broke one metre later. If I could rewind, I would do stuff slightly differently. But it is what it is.
"Charles won by two seconds in the end and the fact he got ahead probably gained him two seconds over the course of the race.
"We couldn't achieve a one-stop as our degradation was too high on the front tyres. That is a weakness for us at the minute.
"Hindsight is a wonderful thing," he added. "We always review things.
"We are doing a very good job. Today was not our day and we didn't get things correct but I wouldn't say we got them wrong either.
"I certainly couldn't have done a one stop as a second car, which is tougher than being the first car.
"Trying to keep with Oscar in the dirty air meant I had to use a lot more tyre. That is just the price I paid."