Sergey Sirotkin's sponsor is driving away from formula one for now.

The Russian driver is gutted that Williams has decided to sign up Robert Kubica instead of him for 2019.

Sirotkin is backed by Russian banker Boris Rotenberg's SMP programme.

"We have decided not to continue our involvement in formula one," Rotenberg said on Thursday.

"We were very unpleasantly surprised by Williams' level of competitiveness and the development of the car this season," he added.

"In these conditions, Sergey did everything he could and we are pleased with his work, so now we'll think about how his career will continue."

Sirotkin did not hide his disappointment.

"It's hard for me to believe what has happened," he said.

"I was told by the team on Wednesday night, but clearly there were meetings with SMP before that. And it was clear that they probably did not want to continue.

"So I already knew the direction it would go in so I could prepare for this step by Williams, but it's still a difficult moment for me," Sirotkin, 23, added.

"I always believed that we can turn things around and that I may be able to keep my place, but nothing changed in the team and so my place is gone," he said.