A European Commission spokesman has confirmed that Aer Lingus has asked it to consider ordering Ryanair to reduce its stake.
Spokesman Jonathan Todd confirmed that the Commission would reply by 17 October.
When the EU regulator blocked Ryanair's takeover bid for Aer Lingus in June, the Commission said it believed it did not have the legal power to order Ryanair to reduce its 25% stake.
Ryanair has since increased its stake to 29.4%.
The two airlines have been in dispute about Ryanair's attempts to call an EGM of shareholders to discuss Aer Lingus's withdrawal from the Shannon to Heathrow route.
