Jack Aitken will make his Formula 1 bow by driving in first practice for the Styrian Grand Prix on Friday.
Aitken, who races for Campos in F2, will replace George Russell for the 90-minute session.
Williams' head of vehicle performance Dave Robson said: "On Friday we will give Jack Aitken his first experience of the FW43 as he drives George’s car during FP1.
"Jack has been working hard with the engineers in Grove, and we are looking forward to working with him at the track as we continue with our intensive test programme."
Aitken joined Williams from the Renault academy for the coming season and is the team's official reserve driver for this year.
Given that a single positive Covid-19 test for either Russell or Nicholas Latifi could lead to Aitken stepping into a race weekend with little notice, getting the British driver seat time at this early stage in the season makes sense.
"Driving in FP1 is a pretty special occasion," wrote Aitken on social media. "I’ll definitely be driving out of the pit lane with a smile on my face."