Qatar Airways has taken a 9.99% stake in British Airways and Iberia owner International Airlines Group.
The Gulf carrier said it may consider increasing its stake further “over time” although this is not currently intended to exceed 9.99%.
No price on the shareholding was disclosed.
Both IAG and Qatar Airways have a close relationship already as part of the Oneworld alliance.
IAG said it welcomed Qatar Airways’ intention to look to strengthen existing commercial ties.
Qatar Airways group chief executive Akbar Al Baker said: “IAG represents an excellent opportunity to further develop our ‘westwards’ strategy.
“Having joined the Oneworld alliance it makes sense for us to work more closely together in the near term and we look forward to forging a long term relationship.”
IAG chief executive Willie Walsh said: “We’re delighted to have Qatar Airways, one of the world’s premier airlines, as a long term supportive shareholder.
“We will talk to them about what opportunities exist to work more closely together and further IAG’s ambitions as the leading global airline group.”
Non-EU shareholders of IAG, including Qatar Airways, are subject to an overall cap on ownership as a result of the requirement for EU airlines to be majority owned by EU shareholders.
The surprise announcement this morning comes as IAG looks to acquire Aer Lingus.