easyJet’s board said it rejected a fourth takeover bid from Minneapolis-based Castlelake that valued the Luton-based carrier at £4.93 billion ($6.49 billion), but signaled a willingness to engage further by granting Castlelake access to limited commercial information in hopes of securing a higher offer. The company said Thursday that the revised proposal, received Tuesday, “substantially undervalues” the airline and reiterated concerns about the offer’s structure.

“The Board believes that giving Castlelake access to limited commercial information, as Castlelake sought in the letter which contained the Fourth Proposal, might produce a more attractive proposal,” easyJet said in a statement cited by The Wall Street Journal.

The latest offer of 650 pence per share, up from the previous bid valued at roughly £4.74 billion ($6.27 billion) that the board rejected earlier this month, comes after easyJet dismissed Castlelake’s initial approach in early June as “highly opportunistic.” Castlelake, which already holds a 2.14% stake in easyJet and has a commercial relationship with the airline as a lender, has been pressing its takeover campaign as easyJet’s share price has been depressed by rising jet fuel costs and route disruptions tied to the Middle East conflict.

easyJet shares closed Wednesday at 539.60 pence, well below the 650-pence offer, suggesting investors remain uncertain about whether a deal will ultimately be reached at a price the board considers adequate. Under UK takeover rules, Castlelake now faces a July 5 deadline—extended from June 26—to announce a firm intention to make an offer or walk away.

The protracted takeover battle illustrates the pressure on European aviation carriers as geopolitical instability weighs on travel demand and operating costs, creating conditions that acquirers view as a buying opportunity. The deal would also test whether EU ownership-and-control rules can accommodate a U.S.-based investment firm taking full control of a major European budget airline; Castlelake said it would address the regulatory structure if its bid succeeds.