Ferrari insists it has always operated within the rules

Seven teams have written a letter to governing body the FIA demanding answers to a series of questions on the Ferrari engine controversy.

The letter is confidential but BBC Sport can reveal it contains an extensive list of queries about a confidential settlement the FIA has reached with Ferrari.

The FIA admitted on Thursday it had suspicions Ferrari's engine was not always legal in 2019 but could not prove it.

The letter includes a deadline by which the teams have demanded the answers.

It was signed by the same seven teams who on Wednesday issued a collective statement expressing their "strong objection" to the deal between the FIA and Ferrari, saying they would "pursue full and proper disclosure" and "reserved [their] right to seek legal redress".

The teams are Mercedes, Red Bull, McLaren, Renault, Racing Point, Alpha Tauri and Williams. The only teams not to sign it are Ferrari and their engine customers Alfa Romeo and Haas.

The seven have declined requests by BBC Sport to share the contents of the letter, which was addressed to both the FIA and commercial rights holder the F1 Group. But the teams are known to be concerned about a number of topics, including:

why the FIA felt it was unable to prove its doubts about the Ferrari engine's legality

why the settlement it reached was confidential rather than communicated more widely

whether the integrity of the finishing order of last year's championship should be questioned

what the FIA's failure to get to the facts on the question of the engine's legality says about the governing body's ability to police the sport's technical regulations

The FIA said in a statement on Thursday that it was "not fully satisfied" with Ferrari's insistence that the car was legal at all times.

The governing body said it decided not to pursue its case because "further action would not necessarily result in a conclusive case due to the complexity of the matter and the material impossibility to provide the unequivocal evidence of a breach".

The FIA said its rules allowed it to look for a settlement with Ferrari, and for it to be confidential, and pointed towards clauses in its regulations that dictate that such deals are subject to the team in question "cooperating with good faith", "telling the whole truth", and "providing genuine, total and permanent cooperation".

But this has not satisfied the seven teams, who still want their further questions answered.

Red Bull motorsport adviser Helmut Marko told the German newspaper Der Spiegel that the situation could have resulted in Ferrari unfairly finishing second in the championship ahead of his team.

"The behaviour of the FIA is the real scandal," Marko said. "We should actually have instructed Red Bull Racing team boss Christian Horner to sue for $24m in prize money that we would have been awarded for second place in the constructors' standings if Ferrari had been punished accordingly."

Ferrari have insisted throughout that their engine was always legal in 2019.

The issue has arisen at a sensitive time for the FIA and F1 as they are attempting to conclude negotiations over the terms by which the teams will sign up to take part in F1 for the five-year period starting in 2021.

No team is committed to F1 beyond the end of this season and the seven are understood to be concerned about the lack of transparency displayed by the FIA and feel it raises questions about how the sport is governed in the modern age.

Some insiders have drawn comparisons with the 1994 Benetton controversy, when Michael Schumacher's car was found to contain illegal driver-aid software but the team were not punished on the grounds that there was no evidence it had been used.

The FIA's investigation into the Ferrari engine last year followed suspicions from rivals - particularly Mercedes and Red Bull - about the extent of the Italian team's advantage in straight-line speed.

A rule clarification at the US Grand Prix in October explained that interfering with the operation of the mandatory fuel-flow meter in any way that could lead it to exceed the maximum permitted rate would be against the rules.

That same day, Ferrari's run of six consecutive pole positions came to an end and the team did not score a pole at either of the remaining two races in Brazil and Abu Dhabi.

The fuel-flow limit in F1's complex turbo-hybrid engines is effectively a ceiling on the amount of power an engine can produce.

Ferrari team principal Mattia Binotto said that apparent differences in the car's straight-line performance seen by rivals following the ruling in Austin were unrelated to the engine and were instead caused by the team changing the way they ran the car by adding more downforce and therefore drag in an attempt to improve its relative lack of cornering performance.