The parent company of British Airways and Iberia has stopped the sales of its stock to investors outside of the European Union.

EU-ownership rules dictate that more than half of an airline's stock must be held by shareholders that are based within the economic bloc.

International Consolidated Airlines Group (IAG) was at risk of breaching that level with 47.5 percent of shares now held by non-EU citizens. Qatar Airways holds almost half of the non-EU allowance with its 22 percent stake in the group.

The filing to the market was released late Monday. Shortly before midday London time on Tuesday, AIG's stock was languishing toward the bottom of the FTSE 100, more than 1 percent lower.

The restriction on British Airways is likely to become even more complicated when Brexit is enacted at the end of next month.

After the U.K. leaves the European Union, U.K. shareholders in airlines will no longer count toward the required 50 percent minimum EU ownership.

IAG, Ryanair and easyJet could reportedly attempt to comply with the ownership rules by ensuring that when British investors sell stock, it is immediately bought by EU investors.