Williams driver George Russell will replace Lewis Hamilton at Mercedes for this weekend's Sakhir Grand Prix.
Hamilton has been forced to withdraw from the race in Bahrain after testing positive for Covid-19.
Williams reserve Jack Aitken will replace Russell.
"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," said 22-year-old Russell.
- I was wrong about halo and I'm happy to admit it - Jolyon Palmer column
- 'A dream I've been chasing since I was three' - Schumacher to join Haas for 2021
- Pirelli backs tyre design despite criticism
"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. I'm really looking forward to the opportunity and can't wait to get out on track this week."
Mercedes have guided Russell's career since 2017 and he was a Mercedes reserve driver in 2018 before they released him to Williams on a three-year contract from the start of the 2019 season.
He is highly rated by the team as a candidate for a potential future drive.
The Briton has tested several times for Mercedes but running him in Bahrain this weekend - as well as potentially the season-closing race in Abu Dhabi a week later if Hamilton does not recover in time - is a good opportunity for the team to assess his development in a competitive environment.
Mercedes inquired as to Williams' position on Russell in the summer before they confirmed Valtteri Bottas for the 2021 season, but were told they were not prepared to release him.
However, that was under Williams' previous management before their sale to the investment group Dorilton Capital in August.
"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 a detailed understanding of the 2020 tyres and how they perform on this generation of cars," said Mercedes chief Toto Wolff.
"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 [Bottas], 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."
Aitken, 25, will make his F1 debut when he takes to the grid this weekend.
A practice driver at the Styrian Grand Prix earlier this year, the Briton has also competed in Formula 2 this season, earning two podiums and eight other points finishes. He finished fifth overall in F2 last season.
"I'm absolutely over the moon to have the opportunity to make my debut with Williams this coming weekend and I am extremely happy for George to have his chance too," said Aitken.
"I really mean it when I say I've felt very much at home since I joined Williams earlier this year, so to get my chance to help the team try to achieve that elusive points finish is an extremely satisfying occasion to say the least."
- Anton Ferdinand: Football, Racism and Me: How the John Terry incident drove him to tackle the problem in the game
- Rugby League's Great British Lions: Follow a behind the scenes look at their return tour down under