* Mercedes accuse FIA of double standards
* Team risk heavy sanction for breach of testing rules
* Verdict due by Friday
By Alan Baldwin
PARIS, June 20 (Reuters) - The threat of sanctions hung over Lewis Hamilton and Nico Rosberg's Mercedes Formula One team on Thursday as an International Tribunal prepared to rule on their 'secret' Pirelli tyre test.
Tribunal president Edwin Glasgow, a prominent British lawyer, told those present in the Place de la Concorde headquarters of the International Automobile Federation (FIA) that a verdict would come by Friday.
If found guilty of breaking the strict testing rules, Mercedes could be hit with anything from a reprimand to outright exclusion from the world championship - although that is highly unlikely.
Mercedes team principal Ross Brawn, no stranger to such legal gatherings after years with Benetton, Ferrari and his own Brawn team, said as he left the building that the seven-hour hearing had been "very fair".
"We were pleased to get our chance to put our case that we did not seek to gain any sporting or competitive advantage," said Brawn, whose team are due to race at their home British Grand Prix next week.
With reporters watching and listening in on a video link, the four judges heard the FIA's lawyer Mark Howard accuse Mercedes of gaining an unfair advantage by making their 2013 car and drivers available for a Pirelli tyre test in Spain in May despite a ban on testing.
He dismissed Mercedes' arguments that they had been given verbal approval to use the 2013 car by the FIA's most senior technical figure, race director Charlie Whiting, in two telephone conversations.
Howard said the Briton had no power to authorise the test and permission should have instead been sought from the FIA's world motorsport council and secretary general Pierre de Coninck.
Although track running with a current car during the season is outlawed, Pirelli - who were also summoned to the hearing - are allowed to do a number of tests. They argued that they are not a competitor and therefore outside the regulations that apply to teams.
Mercedes' counsel Paul Harris accused the FIA of double standards in deciding that Ferrari, who had joined champions Red Bull in protesting against his clients, should face no charges for their own test with Pirelli at the same Barcelona circuit in April with a 2011 car.
"The key differences in treatment are plain," said Harris, criticising a sporting body run by former Ferrari team boss Jean Todt.
"Ferrari were allowed to rely on a verbal confirmation from Pirelli that authorisation had been achieved but apparently we are condemned for this.
"Ferrari's dealings with the FIA were non-specific as regards dates, location, names of drivers. They are not criticised but apparently we are," he added.
"Ferrari was even more involved in the actual testing than we were, they booked and paid for the circuit. They are not criticised."
The lawyer went on to say that Ferrari's test in 2013 was not just a Pirelli test and also involved a current driver in Brazilian Felipe Massa.
"One can see from the run sheets ... that in the middle of the day they were doing their own thing," he said. "But they are not criticised. They also exceeded the 1,000 kilometres."
Harris emphasised repeatedly that the test was organised by Pirelli, was conducted in good faith and Mercedes had no control and no knowledge of what tyres were being used.
He said Mercedes should be applauded for helping Pirelli improve the safety of their tyres.
"We did it for a laudable aim," he said.
He apologised for the drivers using unmarked helmets, which he said was to avoid unwelcome attention from fans rather than to avoid discovery, and recognised that had been a mistake because it fuelled suspicions of secrecy.
Harris said Mercedes should be cleared but if that was not to be the case then a reprimand would suffice.
The lawyer suggested that as a last resort, a young driver test scheduled for Silverstone in July could be used to "level the playing field" by barring Mercedes from the three days.
Red Bull principal Christian Horner, who attended the hearing along with chief engineer Paul Monaghan, said there should be some sort of punishment.
"It's not for us to comment on what the penalty should be but usually if you commit a sporting offence there's a sporting penalty that goes with it," he told Sky television.
Ferrari sent two lawyers while Mercedes-powered McLaren and Williams also sent legal representatives.