Williams and Mercedes have agreed to a five-year extension of their Formula 1 engine supply deal, which will take their partnership to 2025.
Mercedes began supplying power units to Williams back in 2014 at the start of F1's turbo hybrid era, with an initial seven-year deal that took it to the end of the current Concorde Agreement.
But Williams will continue to make its own gearbox rather than join Racing Point and take a supply from Mercedes.
"We have enjoyed a wonderful partnership with Mercedes-Benz over the past six years of what was originally a seven-year agreement, and so we are delighted to be continuing working with them for a further five years from 2021," said Williams deputy team principal Claire Williams.
"Mercedes-Benz has been one of the sport's most successful engine suppliers and we believe that they will continue to have an extremely competitive engine package going forwards.
"Over our time with them they have become a real friend of the team, and we look forward to working with Mercedes-AMG HPP in future."
Williams has enjoyed a good relationship with its current engine supplier for a long time, in part because Mercedes motorsport boss Toto Wolff was previously a shareholder and director at the Grove-based team.
That was reflected in the way the Mercedes could conclude a deal to take Valtteri Bottas after Nico Rosberg's sudden retirement at the end of 2016.
"Williams is one of the iconic brands in F1 and we at Mercedes are proud to count them as part of our motorsport family," said Wolff.
"They have been through some tough times recently but that has only served to demonstrate their resilience and strength of character as they battle back to where they belong on the grid.
"I am certain that the outlook for independent teams is bright under the financial regulations that will be introduced from 2021 and we are delighted to continue our partnership with Williams into that new era of our sport."
Mercedes High Performance Powertrains engine chief Andy Cowell said: "Williams has been an exemplary partner since we began working together ahead of the 2014 season and we have enjoyed a number of highlights during those years.
"We have built strong human and technical relationships to the team in Grove and very much look forward to building on them in the years ahead."