Robert Kubica insists he is "not worried" despite Williams' catastrophic start to its 2019 campaign.

Rumours have been flying about in the Barcelona paddock, after the once-great British team's 2019 car was two days late and then woefully off the pace.

Former Williams driver Sergey Sirotkin told Russia's Championat: "When I was in the simulator, the (2019) car was two seconds slower (than the 2018 car)."

Pole Kubica is making his highly anticipated return to F1 this year after eight years on the sideline with serious injury.

But he is now facing a full season at the back.

"I'm not worried," he said in Barcelona.

"It's the way it is. Of course it would be better if we had learned more about our car this week, but the reality is we are late.

"I hope it will not hurt us in the first race."

On Wednesday, the FW42 was an alarming 8 seconds off the leading pace. The drivers slashed that to four seconds on Thursday.

"It's easy to find four seconds when your deficit is eight seconds," Kubica smiled. "We certainly cannot do that again, but we also cannot focus on laptimes at the moment."

When asked what the 2019 car is like, he answered: "In some areas we have been able to take a step forward, but the rules mean that we have had to take small steps backwards as well.

"One thing is for sure: we have to take a bigger step than our competitors do for us to catch up.

"The guys certainly did a good job to build up the car five times faster than normal, but the preparation takes that long for a good reason as well," he added.

The question now is whether Williams can head to Australia next month fully prepared, having had a notably bad start to the season.

"Well, the others didn't start testing from day one just for fun," Kubica said.

"But the situation is as it is. Everyone has their job on the team and I try to do mine as well as possible."