Williams driver George Russell will replace Lewis Hamilton at Mercedes for this weekend's Sakhir Grand Prix after Hamilton tested positive for COVID-19 on Monday.
Williams, which is an engine customer of Mercedes, agreed to release the 22-year-old for this weekend's race and has announced it will replace Russell with its reserve driver Jack Aitken.
Russell has a contract with Williams until the end of 2021, but is managed by Mercedes and has a separate long-term deal with the world champions.
When news broke of Hamilton's positive test on Monday, Mercedes approached Williams over Russell's availability and once a deal was struck the two teams confirmed the driver switch on Wednesday morning.
It is not clear if there is an option in place for Russell's release from his Williams contract to stretch to the Abu Dhabi Grand Prix the following weekend if Hamilton fails to return a negative COVID-19 test in time.
"Firstly, I want to say a huge thank you to everybody at Williams for giving me this opportunity," Russell said. "I might be wearing a different race suit this weekend, but I'm a Williams driver and I'll be cheering my team on every step of the way.
"I see this as a great chance to learn from the best outfit on the grid right now and to come back as an improved driver, with even more energy and experience to help push Williams further up the grid. A big thank you also to Mercedes for putting their faith in me.
"Obviously, nobody can replace Lewis, but I'll give my all for the team in his absence from the moment I step in the car. Most importantly, I wish him a speedy recovery."
Mercedes team principal Toto Wolff added: "First and foremost, I wish to thank our loyal partners at Williams for their collaboration and open-mindedness in making it possible for George to race for Mercedes this weekend.
"The conversations with the team at Williams were positive and pragmatic, and those were the key factors in reaching an agreement."
He has not scored a point in his 36 F1 starts, although that is largely down to Williams' lack of competitiveness. In qualifying he has outperformed his two teammates, Robert Kubica and Nicholas Latifi, at every grand prix since he has driven at.
Russell is familiar with the Mercedes team and has completed four days of official testing with the world champions, with his last outing taking place in Bahrain last year.
He has been tipped as a potential replacement for Valtteri Bottas in 2022 and the two drivers will be matched as teammates this weekend.
"It will not be a straightforward task for George to make the transition from the Williams to the W11, but he is race-ready and has detailed understanding of the 2020 tyres and how they perform on this generation of cars," Wolff added. "George has shown impressive form this year with Williams, playing an instrumental role in their climb up the grid, and I am optimistic that he will deliver a strong performance alongside Valtteri, who will be a demanding reference for him.
"This race will mark a small milestone for us, as we see a member of our Junior programme compete for the works Mercedes team for the first time. We have a job to do this weekend, and the full focus of our sporting efforts will be behind Valtteri and George in order to maximise our points score as a team.
"But, of course, our thoughts are also with Lewis, and we will be supporting him as much as we can to make a rapid recovery during his period of self-isolation."