The seven Formula One teams that do not use Ferrari power units on Wednesday threatened legal action against the International Automobile Federation to force it to publish its report into the Italian team's engines.
Following complaints by rival teams, the FIA investigated Ferrari last year.
They released a statement last Friday, minutes before the end of winter testing, saying they had reached a "settlement" with Ferrari following "thorough technical investigations", the details of which "will remain confidential".
McLaren, Mercedes, Racing Point, Red Bull, Renault, AlphaTauri and Williams responded on Wednesday with a letter in which they said they "were surprised and shocked by the FIA's statement of Friday 28 February regarding the conclusion of its investigation into the Scuderia Ferrari Formula 1 Power Unit".
"We strongly object to the FIA reaching a confidential settlement agreement with Ferrari to conclude this matter." Last season several teams expressed suspicions after Ferrari gained in straight-line speed. The improvement was particularly noticeable after the mid-season break.
During qualifying for the Mexican Grand Prix in October, Red Bull requested clarification from the FIA on the measurement of fuel flow, which is limited by the rules. They named Ferrari to the media.
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In their letter, the teams said they intended "to pursue full and proper disclosure in this matter, to ensure that our sport treats all competitors fairly and equally".
"We reserve our rights to seek legal redress.