Sebastian Vettel has said that he is concerned but not panicked after falling even further behind in the world championship battle in Spain.

The gap to Lewis Hamilton is now 17 points after Barcelona, although unlike in Baku the reason this time is that Mercedes was genuinely faster than Ferrari.

Vettel suspects that it could be because of the tyre compound thickness, which Pirelli changed for this race to reduce the risk of blistering.

"Apparently, the changes that were made did us more harm than it did our opponents," Ferrari's Vettel said.

"Fortunately, there are normal tyres again for Monaco."

Mercedes boss Toto Wolff denied that Pirelli made the change specifically to benefit the reigning world champions.

However, now Sky Italia reports that Pirelli may test the pre-Spanish Grand Prix tyres in the post-race test this week to see how they compare to the new ones.

"In the next few days there will be meetings and we don't know if the same tyres will be used in France and Britain. But if they go back, it will be like admitting the mistake," a source said.

If Pirelli sticks to the plan, Vettel admits that Ferrari has some homework to do.

"We need to understand why we struggled with the tyres more than the others did," he said.

"I think we have the same tyres in France and Silverstone so it's important that we find an answer."

In Ferrari's favour, Mercedes has struggled at certain circuits in past years, including Monaco.

"There are just some tracks that we are not that good at," Wolff admitted. "Monaco, Hungary, Singapore. Part of why that is we know, but part is still a mystery.

"So that's why I'm only partly celebrating. I'm thinking of Monte Carlo."

The Monaco Grand Prix takes place in two weeks' time.