Red Bull launched a protest on Sunday after Mercedes and Formula One tyre provider Pirelli carried out tyre tests following the Spanish Grand Prix two weeks ago.
Testing during the season is forbidden under Formula One rules, and rival teams Red Bull says the extra testing could give Mercedes a technical advantage.
A Ferrari spokesman said it was also seeking clarification.
Pirelli defended the three-day tests conducted at Barcelona’s Circuit de Catalunya with the 2013 Mercedes car and drivers Lewis Hamilton and Nico Rosberg.
Pirelli’s head of motorsport, Paul Hembery, said: “It’s completely regular in that we are allowed to do a 1,000-kilometre tyre test with any team.
“In the world rally championship contract it’s exactly the same.
We can do it with a representative car. We’ve done it before with another team, and we’ve asked another team to do some work as well.”
Pirelli tested development designs for next season and a new tyre it plans to introduce at next month’s Canadian Grand Prix.
Red Bull team principal Christian Horner said the tests had “not been done in a transparent manner” and the team would be seeking clarification.
“A three-day test has taken place with a current car running on tyres that are going to be used in the next Grand Prix and irrelevant of what you call it, that’s testing,” he said.
“They have run a current car with current drivers and current tyres. That’s a clear breach of the sporting regulations and we’ve protested because we want clarity.
“It’s not right for a current team to test a current car on tyres we’re going to be racing on here or in two weeks in Montreal.”