McLaren and Mercedes have announced that they will be reunited in 2021, after agreeing a Formula 1 engine deal that will last until at least the '24 season.
The two parties were last together in 2014, in the first year of F1's V6 turbo-hybrid era, while McLaren waited for its Honda deal to commence in '15.
The announcement means that Renault will head into F1's new rules era without any customers, and with the only engine used by a single team.
Renault and McLaren joined forces in a "marriage of convenience" that was agreed in 2017, after the Woking team decided to part ways with Honda.
With first Toro Rosso and then Red Bull this season moving to Honda, McLaren became Renault's sole customer this year.
In discussing a new deal, Renault was keen to push for closer co-operation with McLaren, potentially sharing intellectual property on non-listed parts in an attempt to cut costs and help both teams make progress while competing against teams that work in partnership with others.
However, McLaren preferred to stick to a straight engine supply deal, and the position of the two teams as direct rivals on the track this year is also likely to have played a part in its thinking.
McLaren also regards Mercedes as the "benchmark" power unit, despite Renault's recent progress.
"This agreement is an important step in our long-term plan to return to success in F1," said McLaren CEO Zak Brown.
"Mercedes is the benchmark, both as a team and a power unit, so it is natural we would seek to secure a relationship with the company for the next phase of our journey.
"This announcement reflects the confidence of our shareholders and is an important message to our investors, employees, partners and fans that we are committed to returning McLaren to the front of the field."
The move means Mercedes will supply four teams for the first time since 2016, when it had Manor Racing as well as current customers Williams and Racing Point (then Force India).
"Although the two brands share a prestigious history, this new agreement is all about looking to the future and beginning a new era of power unit supply for the years ahead," said Mercedes boss Toto Wolff.
"McLaren have been putting in place the building blocks of their revival over recent seasons, including impressive performances this season with Renault power.
"We hope that this new long-term agreement marks another milestone for McLaren as they aim to take the fight to the sport's top teams, including our Mercedes works team."
Renault F1 boss Cyril Abiteboul told Autosport he saw the loss of its sole customer as a "neutral" development, and while Renault was happy to agree a new deal - based on the close co-operation outlined above - it does not see going it alone as a negative.
"Since our partnership began, McLaren has gone from ninth to fourth position in the constructors' championship," he said.
"We can therefore consider this a very successful relationship.
"However, while looking beyond the terms of the current contract, which concludes at the end of 2020, it was apparent that Renault and McLaren have different ambitions for the future.
"Each of the different elements of this decision have been carefully evaluated over the past few weeks.
"2021 will be a crucial season for all teams and it is important for us to have a precise and clear view of the strengths and ambitions of our competitors going forward.
"This decision is in line with Renault's vision to become a works' team, with a goal to return to the front.
"Renault will continue to honour its commitments to McLaren Racing next season, as has always been the case over our long history of engine supply."
McLaren's and Mercedes' previous relationship ran from 1995 until '14 and yielded three drivers' titles.