Williams have reaffirmed the reason for retiring Robert Kubica's car during the Russian Grand Prix after questions were raised by the Polish driver's chief sponsor.
Kubica was called into the pits shortly after team-mate George Russell crashed into the barriers under the VSC following suspected brake failure.
That raised questions at Orlen, who are estimated to have put £10million behind Kubica to secure a race seat at Williams this year and company spokesperson Joanna Zakrzewska posted on Twitter: "Following the retirement of Robert Kubica from Sunday's Russian GP, we have officially asked the ROKiT Williams Racing team for an explanation. Based on that we will take actions to enforce contractual obligations."
Although Williams did not directly address Orlen's concerns, a statement released following Zakrzewska's made an attempt to further explain the team's thinking in Sochi.
"We found an issue with the wheel nut retainer on George's car, which led to the front-right wheel not sitting perfectly," said senior race engineer Dave Robson.
"This caused a lock-up under braking. The design is very mature and well-proven. The remaining inventory will be inspected thoroughly, and we do not anticipate a repeat of the issue.
"Unfortunately, we were forced to retire Robert's car due to the amount of accident damage we sustained in the Singapore-Russia back-to-back races in order to protect ourselves going into the next events.
"The team has worked extremely hard to ensure race quantities have improved ahead of Japan and the final races."
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