Aitken has spent the past three years as reserve driver for Williams, and made his first grand prix start in the 2020 Sakhir Grand Prix deputising for George Russell, who in turn replaced a COVID-struck Lewis Hamilton at Mercedes.
He maintained his relationship with the Grove-based squad even after ending his full-time single-seater career in 2021 in favour of a switch to endurance racing, but their agreement will not be renewed for 2023.
"We’ve mutually decided to part ways," Aitken told Motorsport.com ahead of his IMSA WeatherTech SportsCar Championship debut in this weekend's Rolex 24 at Daytona for the Action Express Cadillac team.
"I kept an involvement last year, which was great. I really like the team and I’m based half-an-hour away from the factory, so it was very convenient to do simulator work and I was happy to help and attend some races.
"But my calendar is too busy these days. I want to focus on my racing programme and on where my career is going, which is in sportscar racing.
"They’ve been very supportive of that, and equally they’ve got younger guys coming through their academy, and they want to give them seat time and time in the sim. It just made sense."
Aitken, 27, said he has no regrets about falling short of his goal to become a full-time F1 driver and is now looking forward to establishing himself in the top tier of sportscar racing.
"It’s a great thing to chase, and I had a great time doing it," said Aitken. "I was one of the lucky few not just to start a race but just to drive the cars. Of course that wasn’t what I was there for, I was there to try and be on the grid full-time and become champion, but for whatever reason, with the way my single-seater career went, it didn’t work out, and that’s fine.
"I’m very happy chasing something in sportscars now and I’m lucky and I can ride that wave, and I’m excited to see how far I can take it."
Aitken is sharing the AXR Cadillac V-LMDh with team full-timers Pipo Derani and Alexander Sims for this weekend's Rolex 24, and says he is relishing his first experience of racing in IMSA so far.
"I have two very experienced teammates, especially in Pipo’s case, so it’s been quite easy for me," he said. "They’ve taken me in and been very welcoming.
"It’s a slightly different culture to what I’m used to in Europe, but I’m really enjoying it and I’m looking forward to seeing this style of racing because everyone has told me what a good series IMSA is to be involved in."
Aitken is confirmed as AXR's third driver for the four races that make up the IMSA Michelin Endurance Cup, but says he is also working on adding an additional programme in Europe to his schedule this season.
He wouldn't be drawn on whether he would race again in the Le Mans 24 Hours following his debut in the La Sarthe classic last year driving for the Algarve Pro Racing LMP2 team.