"Following Christian Klien's decision to pursue other options within Formula One, Red Bull Racing has agreed to release him from his contract with immediate effect," they said in a statement.

Doornbos, the team's test and reserve driver will step in alongside Briton David Coulthard for the final races in China, Japan and Brazil.

The 24-year-old competed in eight races for Minardi, now Toro Rosso, last year. He scored no points.

"Robert is the logical replacement," team boss Christian Horner said. "He will start preparing for his Red Bull Racing debut at the team's test in Jerez (Spain) this week."

Klien had already been told there will be no place for him at Red Bull next season, with Australian Mark Webber joining from Williams.

He was offered the opportunity to switch to a Red Bull-sponsored team in the U.S. CART series but rejected that possibility at the weekend's Italian Grand Prix.