Pirelli has admitted that some teams have "exaggerated" downforce values as the Formula 1 supplier sets its tyre operating guidelines before grands prix.

In Barcelona, the Italian marque's popular decision in 2017 to allow teams to use lower minimum tyre pressures came to a halt in Friday practice.

Germany's Auto Motor und Sport has a theory as to why that happened.

The report said that Mercedes have struggled on the lower pressures in 2017, after utterly dominating in previous years with higher pressures.

Then, ahead of the Spanish Grand Prix, Pirelli increased the mandatory pressure guidelines after analysing downforce data provided by the various teams.

"The suspicion in the paddock is that Pirelli is deliberately being fed excessively high downforce data in order to keep tyre pressures high. It is said to be always the same two teams who deliver the wrong data," said the report.

Pirelli's Mario Isola said: "It is true that some of the downforce values prove in reality to have been exaggerated.

"But such a policy would have little effect, because we can always change the pressures if we feel it is safe to do so."

The championship continues this weekend with the Monaco Grand Prix.