LONDON/BRUSSELS: The European Union is preparing to tighten its scrutiny of airline ownership rules in a move that could complicate U.S.-led takeover bids for British low-cost carrier easyJet and reinforce restrictions on foreign control of European airlines.

An EU official told Reuters the review, expected in the autumn, aims to clarify what corporate ownership structures are permitted to ensure effective control of regional carriers remains within the bloc.

"This is to ensure that foreign investors don't have full control," the official said, speaking on condition of anonymity because of the sensitivity of the matter. "We need to make sure we have sufficient headroom when it comes to control."

The review comes as U.S. investment firms Apollo Global Management and Castlelake compete to acquire easyJet, a contest that could test EU rules requiring majority European ownership and control of airlines.

Earlier this month, easyJet backed Apollo's £5.7 billion (US$7.65 billion) offer, which topped Castlelake's £5.5 billion bid. Neither bidder has publicly explained how it would comply with the EU's ownership requirements.

The official said Apollo, Castlelake and easyJet had not discussed the details of their proposed transactions with European regulators.

The prospect of tougher oversight weighed on investor sentiment, sending easyJet shares down as much as 15 percent, putting them on track for their biggest one-day decline since late 2021.

"The concern is that the industry is on the wrong foot, thinking that we no longer enforce the rules strictly. People will go down the wrong alley because there's a wrong perception," the official said.

Analysts said investors fear the review could delay, though not necessarily derail, a potential acquisition.

"The issue seems to be that while the two bidders seem willing to allow 51 percent of the voting rights to remain with European investors, the EU is concerned that the effective control of the company would be outside of the EU," Goodbody Stockbrokers analyst Dudley Shanley said.

EasyJet is headquartered in Britain but relies on EU operating licenses for its routes and bases across the bloc. Since Brexit, it has capped non-EU ownership at 49.5 percent to remain compliant with EU rules.

Industry insiders have said U.S. buyers could seek economic control while using EU proxy structures to satisfy current ownership rules. Apollo has until Aug. 7 to formalize its offer, while Castlelake's proposal would place 51 percent ownership with a vehicle led by former Malaysia Airlines Chief Executive Peter Bellew, senior aviation executive Mark Breen and potentially other EU investors.

EasyJet, Apollo and Castlelake declined to comment.