McLaren team principal Martin Whitmarsh has hit out at F1's governing body after it appeared to further confuse the situation surrounding engine regulations.

With teams adapting to life without blown diffusers from this weekend's British Grand Prix at Silverstone, Whitmarsh revealed that the exact nature of the new rules had been altered midway through the morning's free practice session. Although weather conditions made the results of the day's two 90-minutes sessions largely meaningless, the McLaren boss claimed that users of the Renault engine - which include title rival Red Bull Racing - had been allowed a concession over throttle usage under braking.

Whitmarsh confirmed during the post-session press conference that he, and many of his fellow principals, had expected the rules to mandate that throttles would close under braking, and was clearly irritated that the situation remained unclear.

"There has been a negotiation and, as I understand it, Renault's throttles are 50 per cent open under braking," he announced, "I think that's probably not what most of us expected coming in to this event. That's been a little bit of a revelation that we gathered during the course of the sessions today and we'll try to understand what we have to do [but], when the goalposts are moving part way through a practice session, I think it makes it quite difficult."

Whitmarsh revealed that there had been as many as five previous directives from the FIA in the weeks and days preceding the Silverstone event, but had not anticipated another coming during the event itself.

"We learnt halfway through the [first practice] session that [Red Bull] hadn't lost as much as we expected him to, because obviously the rules are slightly fluid and appear to change by the hour at the moment," he fumed, "I think we were all a little bit surprised when... the regulations changed halfway through P1. That puts many teams a little bit on the back foot this weekend. We're trying to cope with that at the moment."

Red Bull team boss Christian Horner, meanwhile, applauded the FIA's flexibility after Renault successfully argued that its throttles needed to open on reliability grounds.

"We expect the FIA to regulate in a fair and proper manner, and that's exactly what they've done in this case," he countered, entering into a heated debate with Whitmarsh in front of the world's media, "It's a very difficult job for the FIA to pick their way through this, [and] I'm sure there are a lot of conspiracies in the paddock [about] why Red Bull or McLaren are performing, or some cars aren't, but that's just circumstantial at the end of the day."

Horner also insisted that Renault was not alone in being granted concessions, with McLaren's engine supplier Mercedes also being allowed to bend the rules slightly.

"Martin's interpretation is interesting, [as] my understanding is that Mercedes are firing on over-run," he revealed, "There has been a series of technical directives that have happened since Valencia and the latest is quite clear in that engines that have been run in previous configurations the FIA would take into account on an equitable basis. Mercedes argued that the over-running that they currently do was permitted, which was granted, I believe, on certain handling characteristics that it offered on a historical basis, and Renault is no different to that.

"Renault is in a situation as an engine supplier, not just to Red Bull but to two other teams as well, where precedents have been set in 2009 and 2010. That data has been openly available to the FIA and the primary purpose of opening the throttle and, for want of a better word, cold-blowing as it has become known, has been because of two purposes. The primary purpose being the blip on the down-shift and the second being a reliability issue.

"I think there was an expectation coming here [that] a lot of focus has been placed on Red Bull. Do Red Bull have a silver bullet on their car? We don't but, at the same time, we expect the FIA to regulate in a fair and proper manner and that's exactly what they have done in this case. They are the only ones with all the facts. They are the only ones with the data. They have looked at it. They have listened to Mercedes' case and allowed Mercedes certain parameters. They have looked at Renault's case and they have allowed Renault certain parameters, [both] based on an historical content if you like, on what is a very, very complex subject that perhaps would have been better dealt with at the end of the season when the exhausts move to a completely different location which will remove an awful lot of the emotion that seems to surround this topic.

"I think the FIA have responded in a right and correct and equitable manner as all the engines aren't the same. They operate in different ways. They have different control codes. They are the only ones that are privy to all that information."