Budget airline Easyjet has launched a legal challenge against Air France's takeover of Dutch carrier KLM.
The takeover will restrict consumer choice, Easyjet claims
The British no-frills carrier said it had asked the European Court of Justice (ECJ) to annul the deal on the grounds that it restricted consumer choice.
Easyjet chief Ray Webster said the European Commission - which regulates corporate mergers - should not have allowed the takeover to go ahead.
"The interests of consumers must always come first," he said in a statement.
The Air France-KLM takeover, completed earlier this month, created Europe's biggest airline by traffic.
The two airlines cleared a final hurdle when the European Commission said the deal could go ahead on condition that they give up 94 of their combined daily take-off and landing slots to rivals.
But Easyjet argues that there are still too few available slots in Paris, and that Air France continues to dominate the wider French market, pushing up prices for French consumers.
"There are a number of significant barriers to entry that prevent any degree of network competition to Air France," Mr Webster said.
The latest legal challenge ups the ante in a bitter competitive struggle between Air France and Easyjet, which ranks as France's second biggest operator after the national airline.
Last week, Easyjet took legal action against French airport landing slots regulator Cohor for allegedly favouring Air France.
The European Commission defended its decision to approve the Air France-KLM takeover.
"We have adequate remedies in place to facilitate new entries. We think this is a very pro-competitive and pro-consumer result, and we stick by that," a spokesman said.
Air France chairman Jean-Cyrile Spinetta said Easyjet's legal challenge was groundless.
"These accusations are untruthful," he told French radio.