Lewis Hamilton's new Mercedes has been called "illegal" by Red Bull team principal Christian Horner as a rules row threatens to engulf the new Formula One season.

Just 10 days before the first race, Hamilton’s Mercedes team unveiled a radically revised machine on the opening day of the final test in Bahrain.

The focus is on Mercedes’ virtually non-existent sidepods, designed to provide greater airflow, improving downforce, and effectively making the car go faster.

Mercedes’ innovative design comes after the biggest technical overhaul of the sport’s rulebook in a generation.

However, Horner questioned whether his rivals have bent the rules.

In an interview with German publication Auto Motor und Sport, Horner said: "From our point of view, Mercedes went a step too far.

"That doesn’t correspond to the spirit of the regulations. For us, these wings are illegal."

Rival teams could protest the Mercedes at the opening round in the Gulf Kingdom on 20 March.

The car could also be outlawed if eight of the grid’s 10 teams – the so-called super majority – believe the concept is not within the spirit of the revamped regulations.

F1 motorsport boss Ross Brawn, who has spearheaded the rule change, admitted he was caught off-guard by Mercedes’ new design.

Brawn said: "There are some very extreme interpretations of the regulations which could lead to a lot of debate. We did not anticipate the Mercedes concept.

"The regulators of the sport know what’s going on – I think they’re OK with it so far – but a team may come in and raise an objection that the FIA hadn’t considered, and then you have a problem."

But Mercedes team principal Toto Wolff said he is confident the team’s car is legal.

"When you go down a specific development direction, the FIA scrutinises it, and you make them a part of the process," he said.

"We were keen in not running alone, but being in touch with the FIA, and that is why I think it will be OK.

"It is clear that when you come with an innovation it creates the kind of debate that we are having here. That was expected.

"We have new governance with the super majority. I would have preferred to stay with the old system where if a team comes up with an innovation you cannot take it off the car as long as it is compliant with the regulations.

"But the FIA and Formula One will handle that with diligence and in the spirit of the sport."

F1 enjoyed one of its greatest seasons last year, with Max Verstappen beating Hamilton to the title following a deeply contentious finale in Abu Dhabi.

The fear now is that Mercedes, winners of the past eight constructors’ championships, might have stolen a march over the opposition, resulting in a one-sided campaign.

But Verstappen said: "I am not concerned. We can only focus on ourselves and that is we have to do this year even if the car looks different. There are still many unknowns so I am not worried."

Hamilton’s new Mercedes team-mate George Russell added: "I don’t think it is advantage anybody at the moment. Ferrari probably look the strongest overall.

"It has caught a few people’s eyes this morning, and I guess from my side I am proud to be part of a team that is pushing innovation, but we need to see if it is fast on track."

Times in testing mean very little as the teams trial different strategies. Pierre Gasly finished fastest for AlphaTauri, half-a-second clear of Ferrari’s Carlos Sainz. Hamilton was 11th. The final test concludes on Saturday.