DOHA (REUTERS) - Red Bull are concerned about the legality of the rear wing used by Formula One rivals Mercedes and will protest if they see something suspicious at this weekend's Qatar Grand Prix, principal Christian Horner said on Friday (Nov 19).

Horner spoke after stewards rejected a Mercedes request to review a decision not to punish Red Bull's championship leader Max Verstappen for an incident with Lewis Hamilton at the previous race in Brazil.

Mercedes lead Red Bull by 11 points in the constructors' championship, with Verstappen 14 clear of Hamilton in the drivers' standings with three races remaining.

Red Bull have long had suspicions about the Mercedes rear wing, with Verstappen feeling it with his hands after a Saturday sprint race in Brazil and drawing a €50,000 (S$77,000) fine.

The FIA introduced new flexibility tests after Azerbaijan in June following suspicions some teams were bending the rules with wings that passed static inspection but worked differently at speed.

Horner spoke of 'score marks' from flexing apparently visible on the end-plates of the wing.

"I think this is something even more advanced... it's hidden in the way that it operates so that it's harder to spot from a camera," he said.

"You can see the straight line performance since Hungary (in August), and particularly in the last two grands prix, has gone exponential. And I think that obviously concerns us."

Asked whether Red Bull were prepared to protest, Horner replied: "Make no bones about it, if we see it on the car here it will be protested."

Andrew Shovlin, Mercedes' head of trackside engineering, rejected the suspicion.

"We've had a look at it (the wing) and there are no score marks," he told Sky Sports television.

"From our point of view we're absolutely happy with what we've got on the car. We've invited the FIA to look at it as much as they want, they don't have any issue with what we've got," he added.

"We'll work out what's fastest, we'll stick it on the car and we'll not take advice from another team."

Horner said Red Bull technical head Adrian Newey and chief engineer Paul Monaghan had been discussing the matter with the governing body.

Horner said the wing was likely to be less of a factor in Qatar than at Saudi Arabia's Jeddah street circuit and Abu Dhabi, the last two races of the season.

"You could have a situation like Brazil where the car (Mercedes) is quite simply unraceable," he said of his rivals' straight line speed.