Piastri’s best time would have seen the Australian third for Sunday’s race but was deleted for track limits at Turn 6.
Stripped of his best lap, the 23-year-old was classified seventh fastest at the end of the session.
Piastri was frustrated and embarrassed by the loss of his lap and questioned how he could have run wide.
McLaren requested evidence of the breach, which team boss Andrea Stella suggested was less than clear.
“We launched a protest, which has been acknowledged but not been actioned yet, because we want to have the possibility to continue the conversation,” he explained.
“Our approach to racing is we don’t want what we don’t deserve. But when the penalty is so harsh, then in the interest of sport – it’s not in the interest of McLaren – there needs to be clear evidence.”
The Italian argued that the images McLaren had been provided were, in his opinion, inconclusive.
“We sought clarification in particular because we wanted to look at the evidence whereby the car was beyond the track limits beyond any reasonable doubt,” he stated.
“I cannot say that the beyond reasonable doubt is satisfied.
“There’s a couple of principles. One is that the system used needs to have adequate resolution, and the second one is that the methodology used for one car needs to be applicable to all cars.
“If you use a helicopter view for a car, you need to use the helicopter view and it needs to be available for all cars.
“We are normally very supportive of the FIA. We always recognise that everyone is trying their best but in this case, we couldn’t agree that the car is beyond the track limit beyond any reasonable doubt, and satisfying the two conditions I said before.
“The discussion is still ongoing.”
According to Stella, shadows and a lack of resolution in the imagery provided create doubt in their mind as to whether Piastri had indeed left the confines of the circuit.
A total of four laps were deleted in qualifying, with Yuki Tsunoda, Pierre Gasly, and Charles Leclerc also having times deleted for track limits at Turn 6 – the second of two left-hander’s as the circuit descends the hill in the middle of the lap.
“What we used is a fixed camera and a helicopter view,” Stella explained.
“In both cases, and like I say, we really appreciate the massive effort to enforce the regulations, but I don’t think we can say that the resolution is adequate.
“We had a case last year in Qatar, when Lando [Norris] was spotted beyond the track limits by the helicopter view and there’s clear resolution and accuracy; the car is outside, thank you very much, we move forward.
“But in this case, it’s just everything blurred and affected by the shadow,” he added.
“It’s quite a lot to come here, compete, put together qualifying laps, and when the penalty is so severe, like having the lap deleted.
“We need to make sure that the penalty is enforced beyond any reasonable doubt.”
Stewards however ruled that the protest was inadmissible arguing that it lack critical information.
That included the regulation that was alleged to have been breached, and against whom the protest was lodged.
“It was addressed to the Clerk of the Course and not to the Chairperson of the Stewards,” stewards outlined in their ruling.
“It did not specify any relevant regulations.
“It did not specify against whom the protest was lodged.
“It did identify the concerns of the protesting party.”
Those points resulted in the stewards deeming the protest inadmissible.
“The subject matter of the protest was a decision (deletion of a relevant lap time in accordance with Article 12.4.1 e) of the FIA International Sporting Code) taken by the Stewards. A decision of the Stewards is not open to protest (Article 12.3.3 ISC),” stewards noted.
“Additionally, the protest does not meet several of the required criteria for the admissibility of a protest.
“The Stewards therefore reject the protest as it is inadmissible.”