Williams' driver lineup for next year is not set in stone.

The struggling British team is set to lose Lance Stroll to Force India, but it was believed Williams would need to at least retain Sergey Sirotkin and his backing by the Russian bank SMP.

But SMP boss Boris Rotenberg told Tass news agency: "There is no agreement for next season, first we are still in this season."

Also unclear is whether reserve driver Robert Kubica is under consideration.

"I don't want to be in a situation like last year," the Pole told Eleven Sports.

"Everyone should have their deadline and their honour. There is no decision yet, but it should be quite soon," Kubica added.

"I will not wait until December."

He added that he wants to return to racing in 2019, and said switching to another series is possible.

"The situation when it comes to F1 is not easy, but there are also opportunities to be somewhere else. For me it's quite transparent.

"I am waiting but if nothing happens, I will make my own decision," said Kubica.