Sergio Perez will leave Racing Point at the end of the season, paving the way for four-time world champion Sebastian Vettel to join from Ferrari.
Vettel's arrival has not been confirmed but it is known that Racing Point, which will become Aston Martin in 2021, have offered him a contract.
Perez, 30, had previously insisted he was not leaving, pointing out that he had a contract running until 2022.
"It hurts a bit as I bet on the team during very rough times," Perez said.
The Mexican played a key role in preventing the team going out of business when it was known as Force India in 2018.
"We managed to overcome obstacles and I am very proud of saving the jobs of several of my team-mates," Perez added.
"I'll keep the memories of the great moments lived together, the friendships and the satisfaction of always giving my all."
He added: "To the current administration, led by Lawrence Stroll, I wish nothing but the best in the future, especially with the upcoming Aston Martin project.
"I don't have a plan B. My intention is to continue racing here [in F1] but that would depend on finding a project that motivates me to continue giving my 100% each lap."
Perez has been linked with both the Haas and Alfa Romeo teams in recent weeks.
German Vettel, 33, is expected to be announced at Racing Point in the coming days, with the Tuscan Grand Prix weekend at Italy's Mugello circuit starting on Thursday.
Racing Point team boss Otmar Szafnaeur praised Perez, who is known as Checo, saying: "Checo's established his reputation as a tenacious racer and together we have enjoyed some fantastic moments.
"If there was ever a sniff of a podium, Checo was ready to pounce, and those five podiums represent some of the finest days in the history of this team. Outside of the car, Checo is a true character and a good friend, and it's been a pleasure working with him for such a long time."
Andrew Benson, Chief F1 writer
These are dramatic and fast-moving times at the Racing Point team, and signing Vettel - as they are believed to have done - is a big statement of intent for the future.
Team owner Lawrence Stroll is a Canadian billionaire businessman who has a reputation for getting what he wants and not minding who he treads on to get there.
His treatment of Perez in this scenario is in the best ruthless traditions of a sport famously dubbed the "Piranha Club" by former McLaren boss Ron Dennis.
It was Perez who triggered the administration process that allowed Stroll to take over the team in the summer of 2018 - and his move, which ousted former owner Vijay Mallya, was co-ordinated with senior team personnel.
Only last October, Stroll gave Perez a new contract for another three years with the team. Perez seemed an ideal fit for a midfield outfit - he was quick enough, a solid points scorer, and he brought with him sizeable funding from his Mexican backers, including Carlos Slim, the world's second richest man.
But Ferrari's decision to dispense with Vettel before this season had even started changed the landscape.
Vettel had been expecting to enter negotiations over a new contract with Ferrari and was caught by surprise when team boss Mattia Binotto rang him up and said he was surplus to requirements.
Racing Point's 2020, meanwhile, has been packed with controversy.
Their car angered rivals when it was unveiled in pre-season testing in February because it was clearly a carbon-copy of last year's Mercedes, a fact the team made no attempt to disguise.
When the season made its belated start after the peak of the coronavirus crisis had subsided, the car was protested by Renault, and Racing Point were found guilty of illegally copying Mercedes' 2019 brake ducts, and docked 15 points and fined 400,000 euros.
Rivals protested against that verdict, seeking further clarifications of the rules, and the last of these protests was withdrawn by Ferrari only last weekend after the confirmation by governing body the FIA of rules intended to prevent such a close copy of a rivals' car being legal in the future.
In the meantime, the Racing Point has emerged, on average, as the third fastest car of 2019. The team have not capitalised on its performance as they might have, but Perez's team-mate Lance Stroll - Lawrence's son - lies fourth in the championship, as do Racing Point in the constructors' table.
Lawrence Stroll took over Aston Martin earlier this year and has grand ambitions to turn around the famous British sports car company, which has stumbled from one financial crisis to the next.
Renaming his F1 team after the historic brand is one part of that plan, and Vettel has clearly been sold on the proposals laid out for him by the team's owner.
Vettel's reputation is not what it was, after three mistake-prone years and being usurped at Ferrari by Charles Leclerc. And if he wanted to stay in F1, it was his only hope of a semi-competitive car. But he will believe he can help the team make even more progress over the coming years.