Sebastian Vettel has diluted the conspiracy theory about why Pirelli's tyres suddenly changed ahead of the Spanish Grand Prix.

Some, including Red Bull's Dr Helmut Marko, mused openly that Mercedes successfully lobbied to have the tread of the tyres made thinner.

The result was Ferrari suddenly struggling last weekend in Barcelona, while Lewis Hamilton went on to win and extend his championship lead.

Yet after testing the old versus new tyres back to back in Barcelona on Tuesday, Ferrari driver Vettel backed Pirelli's decision.

"If we had used those [old tyres at the grand prix] it would have been worse. They blistered very seriously and could have caused serious problems, the German is quoted as saying by La Gazzetta dello Sport.

"It was right to bring the thinner tread.

"Our mistake was that we could not get the wear out of them that our rivals did, and it's up to us to react and work on it.

"We have some ideas and now we need to turn them into reality."

The season continues next weekend in Monaco.