Controversy erupted at the Monaco Grand Prix on Sunday after it emerged that Mercedes, who swept the front row in qualifying for the Formula One race, carried out a secret tyre test with Pirelli last week.
Red Bull team principal Christian Horner, who found out 'second hand' only on Saturday night, told reporters he would be seeking 'clarification' and suggested Mercedes had gained an advantage.
The Formula One regulations ban in-season testing but Pirelli said their contract stipulated that they could do 1,000 km private tests with a 'representative' car and the three days in Spain were legal.
"What's disappointing is it has been done in not a transparent manner that a three-day test has taken place with a current car running on tyres that are going to be used at the next grand prix," an unhappy Horner said.
"Irrespective of what you call it, that's testing."
Pirelli motorsport head Paul Hembery, already under fire from Red Bull over the quick-wearing current tyres, said the test in Barcelona after the Spanish Grand Prix had not been the first of its kind.
"We've done it before with another team and we've asked another team to do some work as well," he said.
Hembery denied Mercedes, who have struggled with tyre wear during races despite having now racked up four pole positions in a row, could have gained any competitive advantage.
"Absolutely not, no. Because it's no relevance to what's happening here," he told Reuters.
Horner, whose cars qualified on the second row behind the Mercedes duo of Nico Rosberg on pole and Lewis Hamilton alongside, was more sceptical about that.
"Well, they've both cars on the front row of the grid so it's not hurt," he said.
"There is confusion between what is contractually permissible, and what a team is allowed to do in the sporting regs," he said. "It's a situation we need clarification on, and our position is we are going to request that clarification."
There was no immediate comment from the governing International Automobile Federation (FIA).
Asked whether Mercedes had tested with this year's car, Hembery said he did not know. There was no immediate comment from the team.
"In reality, we were looking at next year's solutions and trying a variety of different (things)... Mercedes haven't got a clue what on earth we were testing in reality," said Hembery.
He said the test in Barcelona had been rain-affected and was 90 percent aimed at the 2014 tyres.
Asked why the other teams had not been told in advance, Hembery suggested that would have been counter-productive.
"You know in Formula One that when you start talking about something six months could pass before you found a solution," he said. "There's also another point of looking at it... that things take far too long. In reality sometimes you just have to get on and do it."