Ryanair has vowed to refer Lufthansa's deal to acquire some assets of bankrupt Air Berlin to the European Union competition authorities.
The Irish carrier had previously described the possibility of a deal between the two German airlines as a "stitch up" intended to strengthen Lufthansa and late on Thursday (12 October) it announced it will challenge the agreement by turning to the competition watchdog.
"We will be referring the matter to the EU competition authority in due course," a Ryanair spokesman said, without offering any further details on the timing of the legal challenge.
Ryanair's move came a few hours after Lufthansa announced a €210m (£186.6m) deal to purchase Air Berlin's subsidiaries Niki and LG Walter, as well as 20 aircraft.
Germany's second-largest airline filed for insolvency in August, after Abu Dhabi-based Etihad Airways, Air Berlin's main shareholder, withdrew its financial support after the latter racked up years of losses.
Earlier this week, Air Berlin announced it will stop flying at the end of October, telling employees that flights operating under its airline code "will no longer be possible after October 28 at the latest".
Air Berlin undertook a restructuring exercise last year to stem losses, after accumulating debts for almost a decade. In 2016, the airline also confirmed it no longer owned any of its aircraft, having sold and leased back its fleet. It also announced it would be reducing the number of destinations it flies to from 140 to 70.
However, that did not prevent it from reporting a record loss of €782m (£713m) in the last financial year.
Speaking ahead of the announcement on Thursday, Lufthansa's chief executive Carsten Spohr indicated he expected the deal would receive approval from the European Union by the end of the year, which would allow Lufthansa to stabilise operations within six to nine months.
However, Andreas Mundt, head of Germany's cartel office, said both the European Commission and German authorities would closely monitor the deal.