British Airways-parent IAG on Wednesday filed a formal complaint to Brussels over the UK government's last-minute financial rescue of struggling no-frills carrier Flybe.
"IAG has submitted a complaint to the EU Competition directorate this morning about the state aid that the UK government has granted to Flybe," the group said in a statement.
The European Commission early said it "stood ready" to discuss with London the compatability of the proposed Flybe measures with EU state aid rules.
Conservative Prime Minister Boris Johnson's government on Tuesday agreed to review air passenger duty (APD) paid by the Flybe's customers, while the British airline's shareholders have pledged extra investment.
Neither the government nor Flybe disclosed financial details of the rescue agreement.
IAG chief executive Willie Walsh had already reacted with fury to the news, labelling it a "misuse of public funds".
Employing some 2,000 people, Flybe has failed to turn around its fortunes since it was purchased a year ago by a consortium led by Virgin Atlantic.
The Connect Airways consortium -- which also comprises investment firm Cyrus and infrastructure specialist Stobart -- has seen Flybe struggle owing to weak demand and fierce competition.
Virgin and its largest shareholder, US airline Delta, "want the taxpayer to pick up the tab for their mismanagement of the airline", Walsh said in a statement.
"This is a blatant misuse of public funds," added Walsh, who is soon to step down as head of IAG.
Flybe, which claims it has been weakened also by uncertainties related to Brexit, carries around eight million passengers annually and flies from 43 airports across Europe and 28 in Britain.
The carrier is the biggest operator of UK domestic flights but small UK airlines like Flybe have been suffered recently from volatile fuel costs and a weak pound.
The Flybe rescue stands in stark contrast to the fate of British holiday giant Thomas Cook, which collapsed without government assistance last September.
That caused the loss of 22,000 jobs worldwide and stranded 600,000 holidaymakers abroad.
Independent air transport consultant John Strickland argued that Johnson was keen to boost his credentials in areas largely outside of London that are served by Flybe.
"We have a new government keen to establish its credentials in the UK regions and Flybe is a key regional player," Strickland told AFP.
"That said, the airline has struggled financially under a number of managements for several years and suffers from over-ambitious ordering of aircraft... The airline should logically be smaller than it is."
Strickland added: "Other airlines are incensed at government intervention.
"Speculation about the potential elimination of APD on (UK) domestic flights leads to questions about whether it will simply be increased on other flights to Europe for example -- which would simply push the problem elsewhere."
