UPDATE 1-Motor racing-Kubica's F1 sponsor questions Williams' actions

The Williams Formula One team defended their decision to retire Robert Kubica's car from last Sunday's Russian Grand Prix after the Polish driver's sponsors demanded an explanation on Thursday.

Kubica, 34, has already announced he is leaving the struggling former world champions at the end of the season and Polish state-run oil refiner PKN Orlen are set to go with him. "In connection with the withdrawal of Robert Kubica from Sunday's Russian Grand Prix, we have officially asked the Williams Racing team for clarification," PKN Orlen spokeswoman Joanna Zakrzewska said on Twitter in Polish.

"Based on that, we will take actions related to the enforcement of obligations under the contract," she added. Kubica and the team said at the time that Williams had retired the car, which was well out of the points scoring places, to conserve parts ahead of the long-haul races that close out the season.

"The team decided to stop my car, to take care of the parts that we have on the car. We are not in an easy situation and Japan is around the corner," the Pole said in a team release on Sunday. The retirement in Sochi came shortly after Kubica's team mate George Russell had crashed out.

Williams, whose sole point this season was scored by Kubica in Germany, said Russell's crash was due to a problem with the wheel nut retainer that meant the right front on the British rookie's car was not sitting perfectly. "This caused a lock up under braking," said senior race engineer Dave Robson. "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." Robson said the team had worked hard to improve the number of parts available for the remaining races.

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