Russell has been part of Mercedes’ junior programme since the end of 2016, but required allowance from Williams to make the move up to his parent team and deputise for Hamilton.
The agreement is in place for this weekend’s race at the Bahrain International Circuit, but could end up being extended to cover the Abu Dhabi Grand Prix as well should Hamilton be unable to return.
It will give Russell the opportunity to break his F1 points duck dating back to his debut with Williams at the start of the 2019 season, sampling the fastest car in the series’ 70-year history.
Russell has impressed throughout his F1 career to date despite the limitations of the Williams car, with Hamilton recently praising his displays and calling him a potential future world champion.
Russell has long been linked with a possible move up to Mercedes, but is contracted at Williams until the end of the 2021 season.
This weekend’s race will give Mercedes the opportunity to see how Russell fares against full-time driver Valtteri Bottas, although his first run in the 2020-spec W11 car will only come on Friday in practice.
Russell’s seat at Williams has been taken by the team’s reserve driver, Jack Aitken, who was due to race in Formula 2 this weekend with Campos.
Aitken has previously tested F1 machinery for both Renault and Williams, and made an appearance in practice for the Styrian Grand Prix earlier this year.
Aitken holds a full F1 super licence thanks to his junior record, and will become the first driver of Korean heritage to make a grand prix start.
He finished fifth in last year’s F2 championship, scoring three wins, and currently lies 14th in the standings with one round remaining this season.
It will mark the first time since the 2001 Italian Grand Prix where two drivers have made their mid-season debut at the same race.
Pietro Fittipaldi will also make his first F1 start this weekend, stepping in for the injured Romain Grosjean at Haas.