Qatar Airways has raised its stake in Aer Lingus and BA-owner International Consolidated Airlines Group (IAG) to 15.01% and may consider acquiring more of the company over time.
The move comes as the Doha-based airline undergoes a rapid expansion globally.
It has added new flights to Los Angeles, Sydney and other destinations, and has partnered with other airlines to complement that growth.
Qatar Airways said less than a month ago that it had raised its stake in IAG to just under 12% from 9.99%.
Speaking at a press conference in Atlanta, Qatar Airways' CEO Akbar al-Baker said he wanted to deepen links to London, where British Airways has its main hub.
The investment has helped Qatar Airways deliver on its westbound strategy, the airline said in a separate statement.
Under current rules, foreign investors cannot own more than 49% of a European airline.
"Qatar can tap into IAG's extensive American network at Heathrow and Madrid, whilst with limited exposure to the Australasian market IAG can benefit from Qatar's increased activity there," an aviation consultant said.
British Airways announced plans last week for a new direct route between London and Doha.
Qatar's CEO was speaking ahead of the June 1 start of Qatar Airways' new service to Atlanta.
US airlines have charged that the Atlanta flights will lose money because Qatar Airways lacks a partner airline there that can feed it passengers and fill its large aircraft.
Atlanta-based Delta and Chicago-based United have cancelled flights to the Middle East and warned that trans-Atlantic fares will fall because Qatar Airways and other rivals have added flights in excess of customer demand.
Al-Baker said Qatar Airways would fill the flights because "we have a very superior product and people are craving it."
"This is not the end of it," he added. "We have another five locations to announce soon. We cannot announce them yet because of delays with the Airbus."
The airline has delayed the launch of the world's longest scheduled direct flight - from Doha to Auckland, New Zealand - by two months
