Qatar Airways is understood to have paid around $600 million (£461m) to increase its stake in British Airways’ parent IAG from 21.4% to 25.1%.
Since state-owned Qatar Airways first invested in IAG with a 9.99% holding in 2015, it has gradually increased its stake.
“Our investment to date has been highly successful and the announced increase in our shareholding is evidence of our continued support of IAG and its strategy,” said chief executive Akbar Al Baker.
“Qatar Airways continues to consider opportunities to invest in airlines and support management teams that share our vision to enhance travel opportunities for airline passengers across the globe.”
IAG shares rose 1% on the news.
Qatar Airways also has minority stakes in China Southern and Cathay Pacific. Earlier this month it said it was discussing doubling its holding in Chile’s LATAM Airlines Group to 20% and buying a 49% stake in Africa’s RwandAir.