Hartley wanted to focus on racing

Red Bull announced on Wednesday that Jaime Alguersuari would replace Hartley as the official reserve for Red Bull and Toro Rosso from the forthcoming German Grand Prix.

Hartley is contesting both the Formula Renault 3.5 Series for Tech 1 Racing and the Formula 3 Euro Series with Carlin Motorsport at present, and said his diary had become too congested.

"That's the reason we decided to stop," said Hartley. "I learnt so much doing [the 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."

So far this year Hartley's best racing results have been a pair of fourth places in F3 at the Lausitzring, while a ninth place at Catalunya is his highest finish in the Renault series, leaving him 18th in the standings after seven rounds.