Formula One's governing body put Ferrari, the sport's most successful team, in the spotlight along with Mercedes on Friday in an investigation into alleged breaches of the testing regulations.
The International Automobile Federation (FIA) said in a statement that it had asked the teams to reply to an enquiry "in persuance of the judicial and disciplinary rules".
Both were identified as having taken part in tyre tests for sole supplier Pirelli, despite a ban on testing during the season.
A spokesman for the governing body said the two had been asked to reply to a list of questions and had not yet been summoned to any formal hearing or tribunal.
Ferrari and champions Red Bull submitted protests in Monaco last weekend after discovering Mercedes carried out a test for Pirelli with their current car and drivers Nico Rosberg and Lewis Hamilton in Spain this month.
Red Bull principal Christian Horner suggested Mercedes had gained an unfair advantage from the 1,000-km test in Barcelona that he said was carried out in an "underhand" manner without others knowing.
It subsequently emerged from media reports that Ferrari had also tested, albeit with a 2011 car run by their customer division Corse Clienti, for Pirelli some weeks earlier.
Pirelli have confirmed Mercedes were not the only team to have tested for them this season but had not publicly named Ferrari as being the other outfit involved.
Ferrari have consistently sought a lifting of the ban on testing, however, and team principal Stefano Domenicali said at the weekend the protest was more to establish what was allowed.
The testing furore clouded Rosberg's pole to flag win for Mercedes in Monaco and looks set to rumble on into Canada next week, despite Pirelli shelving plans to introduce revised tyres for that seventh round of the season.
Pirelli motorsport head Paul Hembery said on Friday the test had not been "secret", as some have suggested, and Mercedes did not gain any privileged information from a test that focused on the 2014 tyres.
He did however confirm that Pirelli had supplied further information to the FIA.
The FIA said the approach to Mercedes and Ferrari was for supplementary information in the light of Pirelli's replies to a request for clarification made on May 28.
The governing body is weighing up whether to refer the matter to its international tribunal who could inflict heavy sanctions.
Ferrari, with Spain's twice world champion Fernando Alonso and Brazilian Felipe Massa, are second to Red Bull in the constructors standings. Mercedes are fourth.
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