Robert Kubica will become the first Pole to start a Formula One race when he replaces Canadian Jacques Villeneuve in Sunday's Hungarian Grand Prix.
BMW Sauber said the 21-year-old test driver had been promoted to the race seat after 1997 world champion Villeneuve told them he had not fully recovered from an accident at Hockenheim last weekend.
Team chief Mario Theissen left open the possibility, however, that Kubica might replace the Canadian for more than the one race.
"Jacques has informed us that he is not ready to race," he said in a statement.
"Robert has made an outstanding impression with the team so far, both in testing and on the Friday of the race weekends.
"Now we will see how he performs in a Formula One race. However, we have not yet made a decision on who will drive the car either in the remaining races of the season or next year."
Kubica has been strongly tipped to take 35-year-old Villeneuve's place next season, with the Canadian out of contract at the end of this year.
BMW inherited Villeneuve, son of former Ferrari great Gilles, when they bought the Sauber team last year and left it late to confirm he would be their driver for 2006 after investigating various other options.
Despite that, the Canadian has performed far better this year than last and both sides had appeared to be working well together.
However Villeneuve did himself no favours in Sunday's German Grand Prix when he collided with team mate Nick Heidfeld at the start and then plunged into the tyre barriers at half distance with BMW's senior management watching.
Theissen described it last Sunday as a "totally disappointing weekend".
Krakow-born Kubica raced at the Hungaroring in the junior Formula Renault series in 2001 and had described it as a 'Mickey Mouse' track in an earlier team preview.
His only other visit was as a spectator in 1997 when he vowed that he would never attend another Grand Prix unless he was racing.
The Pole said he was up to the unexpected challenge.
"I had prepared myself for a whole year without actually competing in any races so I'm even happier to get his opportunity," he said. "I'm proud that BMW Sauber has such confidence in me. I'm determined not to let them down."