Promising New Zealand driver Brendon Hartley says a desire to concentrate on his racing prompted him to put his role as reserve driver for the Red Bull and Toro Rosso Formula One teams on hold.
Red Bull announced earlier this week that Spain's Jaime Alguersuari would replace Hartley as the official reserve for both their premier teams from the forthcoming German Grand Prix.
Hartley, currently competing in the Formula Renault 3.5 Series for Tech 1 Racing and the Formula 3 Euro Series with Carlin Motorsport, said his schedule had become too congested.
"That's the reason we decided to stop," Hartley said.
"I learned so much doing F1 duties and it was such a great experience for me to see how everything works, but to continue would have meant zero weekends free until November.
"I really appreciated doing it, but I haven't really had a good result this year so I need to focus on my racing.
"Jaime will have that opportunity to learn now, but I am not sure what options there will be for him to get into the car."
Hartley's best racing results this season have been a pair of fourth places in Formula Three at the Lausitzring.
In this morning's (NZ time) first race of the fourth round of the Renault series at Silverstone in England, Hartley steered his Tech 1 car to fifth from a reverse grid start.
It was his highest finish of the season after a ninth in the opening round in April at Spain's Calunya circuit.
Hartley will have chance to improve tomorrow morning because he will start from the second row of the grid after posting the third fastest time in the Super Pole session.
Fellow New Zealander Chris Van Der Drift, currently sixth in the standings, completed just four laps before retiring.