Whitmarsh, who was replaced by Cowell last year, was offered his old job back but is said to have refused.
Whitmarsh told BBC Sport: “I decided to leave Aston Martin in 2023 and, whilst Lawrence has been very generous, I no longer wanted a role in the team.”
The rupture has been caused by a disagreement between Cowell and Newey over who is in charge.
Newey, an F1 design legend and winner of 12 constructors’ and 13 drivers’ world titles with Williams, McLaren and Red Bull, is leading work on the team’s 2026 car.
Cowell as CEO is supposed to oversee the team but Newey’s status, both in terms of his record, and as a shareholder and the team’s biggest earner by far, is inevitably in a more powerful position.
Newey left Red Bull specifically because he had fallen out with Horner as a consequence of the allegations of sexual harassment and coercive, controlling behaviour made against Horner by a female employee.
Horner was cleared of the charges by two separate internal investigations, but the allegations led to major internal ruptures within Red Bull, who also suffered a decline in performance.
With no sign of an improvement in the internal tensions or performance, Red Bull removed Horner after the British Grand Prix.
In September, the two parties reached a settlement that saw Horner receive a pay-off worth 60m euros (£53m).
At the time of his departure, Newey told colleagues he did not want to be in the same room as Horner, a Red Bull source told BBC Sport.
But there is said to have been a rapprochement between the two and in the summer they attended an Oasis concert together.
However, it is far from certain that Horner will secure a role at Aston Martin.
It may be that Stroll decides he does not need a full-time figure of such high profile, given he is de facto team principal as the overall authority and owner.
Cowell would be the fourth senior leader to leave his role at Aston Martin.
Otmar Szafnauer was sacked in early 2022 and replaced by Mike Krack, who was demoted to chief engineer after Cowell took on his role.
The team are also going through a major technical restructure with seven senior design engineers leaving their roles earlier this month.
The team are hoping for a leap forward in competitiveness next year, when new rules could shake up the grid and they become Honda’s engine partner.
