Robert Kubica has admitted that Williams's drivers are suffering from more than just a lack of pace in 2018.
In Barcelona, Sergey Sirotkin complained that he struggled throughout the weekend with "seat comfort".
"Let's say it this way to not upset anyone - the problem is with seat comfort," said the Russian.
Lance Stroll is believed to be suffering with a similar problem, and Kubica, Williams's reserve driver, confirmed the issue.
"We do not have too much comfort in this car, but it has been known since November," he told Eleven Sports.
"Problems have to be solved in the winter rather than pretending that everything is good to make it look nice politically.
"I do not drive, so I don't say anything else."
The season continues with the Canadian Grand Prix next weekend.
