Mercedes chief Toto Wolff says the team's W14 car is a "nasty piece of work" and its performance is "worse than I ever thought it would be".
The team endured perhaps the low point of their 2023 campaign on Saturday as Lewis Hamilton qualified 13th for Sunday's Miami Grand Prix after being knocked out in Q2, while George Russell took a fortuitous sixth.
Mercedes struggled following the introduction of new design regulations last season, and their decision to stick with the same car concept in 2023 has proven a bad one with the W14 unable to compete with Red Bull's dominant RB19.
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Wolff admitted even after all this time pursuing the concept, the team are still struggling to understand why the car isn't competitive.
"I think that the car is not a nice car, not a good car," Wolff said. "And I wouldn't even be able to point out whether fundamentally… no its everywhere, it's the basic performance of the car, the lack of understanding of the car, the whole span of activities.
"I would say the performance is just really bad."
Mercedes were plagued by bouncing last season and had hoped curing that would lead to an upturn in performance, but that has not proven to be the case.
"It's worse than I ever thought it would be," Wolff said. "We had 12 months on since we were last time in Miami and the car is just marginally better, it is not bouncing on the straight, but that is the only thing that is better than last year.
"The car is not fast enough and we haven't got any comprehension why that is. Our car is P1, P2 in one session and next P6 and P13, it's just not acceptable."
Hamilton strategy error 'makes it even worse'
Hamilton was critical of the team's strategy after Qualifying, having been sent out into traffic as he prepared for a final flying lap, for which his preparations were hampered.
Wolff, who said on Thursday that he remains confident Hamilton will sign a new deal to remain with the team beyond this season, admitted such errors make Mercedes' general struggles "even worse".
"There is not a lot to say (to Lewis)," Wolff said. "The car is simply not fast enough and putting him in a situation on his out lap where the driver isn't able to prepare their tyres makes it even worse.
"Nobody makes a mistake on purpose, we're trying to give them the best position. We have in the past got it wrong many times and got it right many times.
"Today for George and Lewis that really went south and you can see in his first sector that the car simply wasn't there."
Russell benefitted from a premature end to the session caused by a red flag after Charles Leclerc crashed his Ferrari, which prevented Max Verstappen from setting a lap time in Q3.
Leclerc had also made an error on his first flying lap in Q3 that left him behind Russell, despite Ferrari comfortably having more pace than Mercedes.
"I take no enjoyment from finishing sixth because probably it was eighth or seventh," Wolff added.
"And even if it was fifth I took no enjoyment either because it's a lack of comprehension of what it is that makes this car such a nasty piece of work."
The Formula 1 season continues with the Miami GP - watch Sunday's race live on Sky Sports F1 from 7pm, lights out at 8.30pm. Get Sky Sports