It has been confirmed that Aer Lingus has asked the European Commission to consider ordering Ryanair to reduce its stake.
"I can confirm we have received a notice about failure to act and we will reply by October 17," said European Commission spokesman Jonathan Todd.
The Commission said it believed it did not have the legal power to order Ryanair to reduce its 25% stake when the EU regulator blocked Ryanair's takeover bid for Aer Lingus in June.
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-Heathrow route.