Robert Kubica's sponsor has questioned the reason given by the Williams Formula 1 team for the one-time race winner's retirement from the Russian Grand Prix.
Kubica's team-mate George Russell crashed out of the Sochi race during a virtual safety-car period, when he locked-up and crashed as a result of what Williams described as an unspecified issue.
Williams retired Kubica's car one lap later.
This was originally believed to be for precautionary reasons after Russell's retirement, but in its post-race press release the team clarified it had stopped Kubica in order to save parts, given the busy flyaway race schedule.
Kubica had a perfect reliability record prior to the Russian GP, having finished every other race this season until that point.
"We opted to retire Robert soon afterwards in order to conserve parts ahead of the intense flyaway races which end the season," said chief race engineer Dave Robson.
"It's a very disappointing way to end two weeks on the road."
Kubica's own comment in the release echoed that explanation: "After George's DNF, 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."
In a tweet on Thursday morning Joanna Zakrzewska, spokesperson for Kubica's sponsor PKN Orlen, said the company had asked Williams for further explanation.
She wrote: "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 steps to enforce contractual obligations."
A Williams spokesperson declined to comment on the PKN Orlen tweet when approached by Autosport.