Qatar Airways has bought a 9.9 per cent stake in British Airways and Iberia owner International Airlines Group (IAG) becoming its largest shareholder.
The state-owned carrier said it would consider increasing this stake as it expands further into European markets, however this is limited by EU rules on ownership by non-EU companies.
The flag carrier of Qatar and IAG already have codeshare agreements through the Oneworld alliance group of airlines, however both companies said the deal would strengthen these ties further.
IAG chief 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".
The deal comes amid a bid for budget airline Aer Lingus by IAG.
Qatar Airways chief executive His Excellency 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."
Standard Life investment- six per cent
Europacific Growth Fund- 5.3 per cent
Capital Reserch and Management- 5.05 per cent
Templeton Global Advisors- 4.9 per cent
Legal and General- 3.2 per cent
Landsdowne Partners International- 2.2 per cent
Invesco- one per cent
Lansdowne Developed Markets Master Fund- one per cent
*As of 19 January