Red Bull Racing has been handed a late boost in its hopes of minimizing the performance loss caused by the ban on the off-throttle use of blown diffusers, after the FIA agreed to an 11th-hour concession on how it uses its Renault power unit.
Amid ongoing discussions between engine manufacturers and the governing body about how much teams are allowed to continue blowing their exhausts, Red Bull Racing's supplier Renault has been allowed to keep 50 percent of its exhaust flow when the driver is off throttle because of reliability reasons.
The original intention had been to limit teams to just 10 percent - but that was then increased marginally to 20 per cent at 18,000rpm after preliminary discussions with teams.
However, following lobbying from Renault, the engine manufacturer argued that it needed even more throttle use for engine reliability related to the exhaust valves.
The new limit was agreed as late as Friday morning, and it counters a concession handed to rival Mercedes-Benz ahead of Silverstone - which has been allowed to keep firing half its cylinders as engine over run in a bid to ease crank case pressure.
The FIA's decision has provided a dramatic twist to the blown diffuser saga, and could have significant implications on the competitive order because it is understood that Red Bull Racing had only been running at 45 percent throttle flow before the rules clarification was made by the FIA.
In the wake of teams being informed about the concession to Renault during first practice, Red Bull Racing team principal and McLaren boss Martin Whitmarsh clashed in public during Friday's FIA press conference at Silverstone.
Both believe they were being handicapped by the FIA's ruling on their opposition - with Horner adamant Mercedes-Benz was benefiting from the over-run, while Whitmarsh was sure Red Bull Racing would be helped by the 50 percent allowance.
Horner said: "There was a technical directive which effectively turned it all off. That was obviously with reticence by the manufacturers and it has been very much a manufacture issue.
"Certain teams were then allowed to have fired overrun, to fuel their overrun, of which there are also, obviously, secondary benefits through the exhaust plumes and thrusts that that creates, But that was permitted."
He added: "It would be unfair to allow fire overrun and not allow the same parameters for another engine manufacturer. I think it's a very very difficult job for the FIA to pick their way through this and I think all credit to them, they've looked to try and be as fair, balanced and equitable as they decreed that they would be through the technical directive, to come up with the solutions that they have.
"We're not totally happy with the solution that we have, that's for sure. I'm sure Martin isn't with his and I'm sure there are a lot of conspiracies in the paddock that these are the reasons why Red Bull is performing or McLaren is performing, or some cars aren't performing. That's just circumstantial at the end of the day. The fundamentals are that the engine manufacturers have been treated in a fair and equitable manner."
Whitmarsh responded: "When the goalposts are moving partway through a practice session, then I think it makes it quite difficult. I think that with the benefit of hindsight, it would have been better to make changes at year end which I think Christian would agree.
"I think that to do this and to do it a fairly cloudy and ambiguous and changing way inevitably, in a competitive environment, every team feels that it's been hard done by. At the moment, I think potentially a lot of teams will end up making an argument to cold blow. Renault have been in that domain for some time, other teams haven't and don't have that experience but we're talking about a very substantial performance benefit here..."
He added: "It's messy and I think the intention people believed was that we were going to stop exhaust blowing when the driver didn't have his foot on the throttle. I think that was the simple concept but that concept has been deflected and therefore it hasn't been clear. And the fact that these things were only coming out during the course of today is fairly extraordinary.
"But nonetheless, I'm sure we will remain calm and pick our way through but I think it's probably better to make changes to the regulations between seasons, not in season and also make changes to regulations which are clear and unambiguous.
"I think at the moment, a lot of people are clearly getting emotional about the situation and I can understand why: it's frustrating for the engineers not to know what it is that we're allowed to do, because these changes... by cold blowing you're getting 30, 40 points of extra rear downforce in braking and that's quite an attractive thing so if you can do it, then you're going to try and do it aren't you?"
Renault team principal Eric Boullier said that it would be almost impossible for the FIA to please all the manufacturers and ensure that no single team was given an advantage.
"I think if Charlie tries to please every engine manufacturer it will be very difficult to make a level playing field," he said when asked by AUTOSPORT about the situation.
The matter was discussed at length in the drivers' briefing, with several drivers unhappy about the 50 percent limit being introduced mid-session.
FIA technical delegate Charlie Whiting is meeting with the engine manufacturers tonight to discuss the situation.
Mercedes GP team principal Ross Brawn said in the build-up to the British Grand Prix that he hoped the FIA allowances would not benefit an individual engine manufacturer, but said there was a chance it could.
"It is possible," he explained. "I think the FIA is aware of that. Even referencing the old engine maps, if they see some things which are contrary to what they want to achieve, so if for some other reason people were using engine maps in 2009 with a lot of throttle opening, then the FIA will still ask for some explanation of why they are doing it.
"But it is not likely people were doing it because it does use more fuel and does create heat in the system, so you would not do it unless there was some benefit."