British police and German investigators have searched the headquarters of Brookfield Aviation International in surrey, the company is believed to supply half of Ryanair's pilots.
The raid is detailed in an exclusive report in The Times (London) - it is in connection with what a German court has described as an "especially serious case of tax evasion" while naming Brookfield directors James Duffield and David George as suspects.
The UK is chasing unpaid tax and social insurance payments of up to €65m - in Germany the offences that the company is being investigated for can result in jail sentences of up to five years.
It is suggested that Brookfield is a "letter company" which is controlled by Ryanair.
In Germany many of the Irish airline's pilots are registered with the UK agency Brookfield Aviation Limited - and they are self-employed according to Ryanair.
It is reported in the German media that the pilots wear Ryanair uniforms - they were obliged to be on-call 11 months of the year - and that they have no right to refuse shifts.
If they are found to be effectively full-time employees, the company could be required to back-pay unpaid insurance and state pension contribution payments.
"We are aware of the investigation and are assisting the German authorities with same," Ryanair said in a statement to Newstalk.
“This is a matter for Brookfield and its pilot contractors. However, the use of contract staff continues to be both legal and widespread in the airline industry.
"Ryanair requires all of its contractors, including Brookfield, to comply fully with national regulations and authorities. Brookfield have advised us that they expect these enquiries will be resolved satisfactorily in the not too distant future," the airline added.