Qatar Airways has emerged as the owner of a 10% stake in International Airlines Group (IAG), the owner of British Airways (BA) and Iberia.
The Gulf airline is already a member of the Oneworld alliance.
Willie Walsh, IAG’s chief executive, said he was delighted Qatar had become a “long-term supportive shareholder”.
The stake, worth about £1.15bn, gives Qatar closer links with a group that has two major European hubs and strong transatlantic networks.
Mr Walsh said his company would explore what “opportunities exist to work more closely together and further IAG’s ambitions as the leading global airline group”.
Akbar Al Baker, chief executive of Qatar, said: “IAG represents an excellent opportunity to further develop our westwards strategy.”
Earlier this month he criticised European airlines, saying they they “cannot keep up” with the competition posed by Gulf rivals.
The two groups’ route networks are largely complementary, with Qatar strong in southeast Asia, India and the Middle East, while Iberia flies to many destinations in South America.
Qatar said it could increase its holding, but would be limited by the 49% cap on non-EU ownership of European airlines.
Neither group said whether IAG had been aware of Qatar’s stake-building and did not say who the shares had been bought from, or when.
Qatar has more than 130 aircraft and a further 340 on order. It is competing with other Gulf airlines including Emirates, based in Dubai, and Abu Dhabi’s Etihad.
The Doha-based carrier had 22m passengers in the 2013-14 financial year, compared with 44.5m for its larger rival Emirates. IAG carried just over 77m passengers in 2014.
Qatar’s move comes as IAG attempts to complete a takeover of Aer Lingus for about £1.3bn.
The company has said it planned to run the Irish airline as a separate business with its own brand, management and operations.
IAG was formed when BA and Iberia merged in 2011. It has about 430 aircraft and 60,000 employees.
Shares in IAG closed down 3.5% at 544.5p on Friday, making it the biggest faller on the FTSE 100.
The stock has risen by more than a third in the past 12 months and the company is valued at £11.4bn.