The first rumours that Robert Kubica could be ousted by Williams have emerged.

The Pole has struggled alongside George Russell on his return to F1 after an eight year absence with a permanent arm injury.

Kubica, 34, has been questioning whether his car is identical to Russell's, so for Barcelona the pair's respective chassis have been switched.

"I think the differences between the cars was a problem with a sensor," Russell said.

"This weekend I am using Robert's chassis and Robert has the new one."

But Russell remained significantly faster in the second Friday practice session in Barcelona.

"The main problem that I had in Baku is still there," Kubica told Eleven Sports.

As for Williams' upgrades this weekend, Kubica said: "We brought improvements to the parts that we already have.

"Some work and some ... unfortunately we did not achieve the results we expected."

Le Journal de Montreal claims that Canadian Nicholas Latifi, the Williams reserve driver whose father is a billionaire, could replace Kubica before the end of the season.

"According to our information, it is time for major decisions within the British team," the newspaper claimed.

"Apparently Kubica has a clause in his contract stating that he could be replaced during the season."