SkyTeam alliance partners Air France-KLM, Alitalia and Delta Air Lines are facing an anti-trust investigation over their transatlantic joint venture.
The European Commission inquiry aims to establish whether the tie-up, which was formed between Air France-KLM and Delta in 2009 before including Alitalia the following year, is leading to anti-competitive behaviour.
It follows similar investigations into transatlantic tie-ups by Oneworld and Star Alliance. Three Oneworld carriers - British Airways, American Airlines and Iberia - agreed in 2010 to offer remedies to allay Commission concerns over their pact.
Under the SkyTeam joint venture the carriers agree to co-ordinate schedules, capacity and pricing.
"This partnership represents the deepest form of co-operation within SkyTeam and aims at the alignment of the parties' commercial incentives," said the Commission as it confirmed the anti-trust probe.
It stated that it would investigate whether any reduction in transatlantic competition, as a result of the tie-up, was proving detrimental to passengers.
But the Commission has closed an earlier anti-trust inquiry relating to co-operation between Air France-KLM, Alitalia, Delta and five other SkyTeam partners, owing to "significant changes" in the various markets covered.
This inquiry had generated concerns over particular routes, including transatlantic services, and the Commission tentatively outlined its objections to SkyTeam in 2006.
Its closure of the proceeding is a "priority-setting" measure given the changes undertaken in the markets affected. But the Commission points out that the termination of proceedings "does not relieve" the SkyTeam partners from "assessing their behaviour" and ensuring compliance with competition law.