Pirelli boss Mario Isola thinks Ferrari will start helping next year with the development of Formula 1's 18-inch tyres for 2021.

In a modified 2018 Renault, Sergey Sirotkin was in the cockpit last week as testing of the low-profile tyres began at Paul Ricard.

"Unfortunately I am not allowed to answer any questions or disclose any information," the Russian driver said afterwards.

But when asked about how the test went, boss Mario Isola told Corriere della Sera: "For the first time, good.

"The behaviour and handling was as expected and there is still some margin with the level of performance," he added. "We are now going through the data and the driver's feedback."

McLaren and Mercedes will also modify 2018 cars to help with Pirelli's 18-inch tyre testing later this year.

"We give the numbers so that the teams can develop their mule cars," Isola said.

"The same information is also distributed to the teams that are not participating."

One of those teams is Ferrari. "It was our choice not to participate," confirmed Ferrari boss Mattia Binotto.

"We are short on time and the resources for the programme so we decided not to take part in 2019."

Isola responded: "I expect them to do it next season, but in the meantime they have done tests for the development of next year's tyres."

He said the 2020 tyres will be tweaked based on what has been learned this year.

"We will continue to work in the direction we have been asked, to revise the compounds and the construction methods for less overheating and a wider usage window."