Red Bull's ever outspoken Helmut Marko has become the first to publicly question the legality of Mercedes' new 'DAS' device.

The 'Dual-Axis Steering' system was the biggest talking point from the second day of testing in Barcelona, after onboard footage showed the front wheels toeing in after Lewis Hamilton pulled the steering wheel towards in on the straight.

Unsurprisingly Mercedes are very pleased with their innovation, making jokes on social media, but the reaction from their rivals looks set to be much less jovial.

“If you actively change the toe of the wheels while driving, the contact surface of the tyres changes and with that – albeit in a very, very small area – the ground clearance.

“But that is forbidden because the system then corresponds to the functioning of a prohibited active suspension," Marko said via Auto Motor und Sport.

Mercedes though remain confident about its legality, claiming the FIA had been informed of the DAS during development.

And another rival, Sebastian Vettel, also doubts the Brackley-based team would run if it was unsure.

“The Mercedes steering wheel? I’ve seen it and the fact that they’re using it makes me think it’s legal," he told FormulaPassion.

“It is probably not as easy for the driver to use as it seems. We’ll see.

“It is certainly new for everyone.”

As for the governing body's view, a spokesperson confirmed to RaceFans they believe the system does meet the regulations per what Mercedes have told them, though did also says they will continue monitoring the situation.

Interesting times ahead.