Ryanair faces the prospect of having to sell all or part of its 29.8% stake in rival Aer Lingus.
The UK Competition Commission is expected to rule as early as this week on the shareholding following an investigation started in June prompted by a request by the Office of Fair Trading.
The inquiry has been held up by a separate European Commission probe into Ryanair’s latest attempt to buy Ar Lingus, which was rejected in February.
The UK competition watchdog has given a deadline of September 5 to publish the report.
But the Sunday Times reported airline industry sources as saying the report could be out by the end of this month.
Ryanair has previous said it would be prepared to sell the shareholding in Aer Lingus if it receives an offer from another European airline.
The Competition Commission issued an initial finding earlier in the year, saying the Ryanair stake was anti-competitive.
Ryanair robustly rejected the findings.