Red Bull is working "day and night" to "uncover the secret" of how Mercedes pulled so quickly ahead in the otherwise intense 2021 title battle.

In recent days, Dr Helmut Marko and his Red Bull Racing colleague Christian Horner cast serious doubt on the legality of Mercedes' latest engine upgrade.

"I don't think Mercedes did anything illegal," Marko insists.

"But something has been weird since Silverstone. So it is now up to us to research why they are suddenly so fast.

"Is it just the engine or is it something else?" the Austrian wondered. "We now have to work day and night to uncover the secret and then counter it as quickly as possible.

"It's up to us."

The implication of Marko's comments is that the FIA is reluctant to probe too deeply into whether rumours of an intercooler 'trick' are true.

But for the world championship fight, it is bad news for Red Bull.

"If the performance difference continues to be the same as in Istanbul, we have a problem," said Marko, 78.

"Of the six remaining tracks, only Mexico and Sao Paulo are in our favour because of the altitude. At the moment, the rest clearly seem to be Mercedes-land."

It is even rumoured that Red Bull may change Max Verstappen's chassis ahead of Austin, so bad was the Dutchman's struggle with understeer.

"Max constantly complained about understeer that we just couldn't get rid of," admitted Marko.

"But we're going to keep fighting. It is also positive just how relaxed Max is in dealing with the situation.

"He remains very calm and only points out the disadvantages that he has at the moment."