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Ilott's ART teammate and championship leader Anthoine Hubert will start in 19th, running wide on his final flying lap and then pulling up at the exit of Turn 6 a lap later.
His stoppage brought out the red flag with under two minutes left on the clock, and the session was not restarted.
Ilott had won the previous race, the Paul Ricard reverse-grid encounter, and followed that up by topping qualifying in Austria.
Like in Formula 2, it was a British 1-2 as fellow ART driver Jake Hughes scored second, qualifying in the top 10 for the first time since his return to the category and missing out on pole by just 0.037s.
Alessio Lorandi showed Trident’s potential by taking third on the grid, 0.067s off pole, ahead of Barcelona polesitter Leonardo Pulcini in his Campos-run car.
The ART of Nikita Mazepin rounded out the top five, having topped the session early on but denied an opportunity to challenge for pole by the red flag.
Pedro Piquet set purple sectors at various points in qualifying but had to settle for sixth in the end, ahead of MP's Dorian Boccolacci.
Ryan Tveter, the practice pacesetter, took eighth, ahead of Gabriel Aubry (Arden) and fellow Trident driver Giuliano Alesi.
New MP Motorsport driver Devlin DeFrancesco, replacing Christian Lundgaard, will start last but was only 0.9s off Ilott's pace.
|1||1||Callum Ilott||ART Grand Prix||15||1'19.209|
|2||4||Jake Hughes||ART Grand Prix||15||1'19.246||0.037|
|4||18||Leonardo Pulcini||Campos Racing||15||1'19.344||0.135|
|5||3||Nikita Mazepin||ART Grand Prix||15||1'19.351||0.142|
|7||22||Dorian Boccolacci||MP Motorsport||16||1'19.453||0.244|
|9||14||Gabriel Aubry||Arden International||15||1'19.502||0.293|
|11||10||Juan Manuel Correa||Jenzer Motorsport||16||1'19.592||0.383|
|12||19||Simo Laaksonen||Campos Racing||15||1'19.606||0.397|
|13||16||Joey Mawson||Arden International||15||1'19.718||0.509|
|14||11||David Beckmann||Jenzer Motorsport||15||1'19.782||0.573|
|15||20||Diego Menchaca||Campos Racing||15||1'19.807||0.598|
|16||24||Niko Kari||MP Motorsport||16||1'19.902||0.693|
|17||9||Tatiana Calderon||Jenzer Motorsport||15||1'19.974||0.765|
|18||15||Julien Falchero||Arden International||15||1'19.995||0.786|
|19||2||Anthoine Hubert||ART Grand Prix||14||1'20.003||0.794|
|20||23||Devlin DeFrancesco||MP Motorsport||13||1'20.191||0.982|
DeFrancesco joins MP for rest of GP3 season
Red Bull Ring GP3: Ilott wins as Mazepin hits Hubert