SMP Racing did not pay in full for its sponsorship of the struggling backmarker Williams in 2018.

That is the claim of a Russian source at Moscow-based Echo Moskwa radio.

Last year, Russian Sergey Sirotkin raced for Williams with backing by SMP Bank's motorsport programme SMP Racing.

"They paid only part of the amount," the source, Marina Maksimowa, told Sportowe Fakty.

Williams is still struggling with its financial backing in 2019. Robert Kubica's sponsor PKN Orlen has been publicly critical of the team and is set to leave along with the Polish driver.

As for SMP Racing, the Russian source continued: "They decided that Williams' results were unsatisfactory and that they should not receive any more money."

The sponsor's dissatisfaction is apparently why Sirotkin left Williams at the end of the season.

"Sirotkin wanted to continue, even with Williams," Maksimowa said. "But they (SMP Racing) told him that there would be no more money."