Mercedes boss Toto Wolff has demanded Formula One's governing body the FIA take robust action over reports Red Bull broke the rules when Max Verstappen beat Lewis Hamilton to last year's world championship.

On the eve of the Singapore Grand Prix - a race which could see Verstappen secure his second title in as many seasons - his controversial maiden triumph is under the microscope amid allegations his Red Bull team overspent.

F1 introduced a budget cap of 145 million (€148m) last year. But Verstappen's Red Bull are facing accusations - reported in Auto Motor und Sport and Gazzetta Dello Sport - that they went beyond that figure, potentially by up to five per cent, which would equate to £7m.

The FIA are assessing Red Bull's finances with a verdict set to be revealed on Wednesday. Sanctions available to the sporting federation range from financial penalties to a deduction of drivers' championship points. The latter could have an impact on last season's result.

Hamilton was denied a record eighth crown when he was beaten by Verstappen at a deeply contentious winner-takes-all finale in Abu Dhabi on December 12. Verstappen took the title by eight points.

Speaking on Friday evening, Red Bull team principal Christian Horner said he was not aware of any breach.

But rival boss Wolff urged the FIA, and its president Mohammed ben Sulayem - elected in the days after the Abu Dhabi fiasco - to follow the rules.

"It is of huge importance for a demonstration that these regulations are policed and I have no reason to believe otherwise," Wolff told the BBC.

"The FIA, particularly Mohammed, has shown a pretty robust stance on enforcing all kinds of regulations.

"So if we are talking now about something big, he will show the same integrity and leadership that he has done before."

If Red Bull exceeded last year's salary cap, that would have also had a potential impact on the make-up of Verstappen's current car which has carried him to victory at 11 of the 16 races staged in 2022.

Verstappen, who has won the past five races, will be crowned champion on Sunday if he triumphs at the Marina Bay Circuit and sets the fastest lap, and team-mate Sergio Perez and Ferrari's Charles Leclerc finish lower than fourth and eighth respectively.

"We are certainly not aware of any breaches," Horner told Sky Sports.

"The accounts were all submitted to the FIA way back in March, so it's been a long process with the FIA and we are in that process as we speak.

"Next week is when they declare their certificates. Our submission was below the cap and it is down to the FIA to follow their process which they are currently doing.

"We are confident in our submission. There are always going to be rumours. I've heard of major breaches but I'm certainly not aware of that."

An spokesperson for F1's governing body said: "The FIA is currently finalising the assessment of the 2021 financial data submitted by all Formula One teams.

"Alleged breaches of the Financial Regulations, if any, will be dealt with according to the formal process set out in the regulations."

On track, Hamilton topped a practice session for the first time this season when he went fastest in the opening action of the weekend.

Hamilton finished fifth later in the day - six tenths adrift of Carlos Sainz who set the pace for Ferrari. Verstappen, who turned 25 on Friday, was fourth, a third of a second back.