The carrier's board said the "unsolicited, indicative and conditional" offer, Castlelake's third, was "highly opportunistic" – reiterating its stance from earlier this month when Clearlake disclosed its interest.
EasyJet said the offer, which at £6.25 per share amounts to £4.74 billion, came while easyJet's share price remains "temporarily depressed" owing to the conflict in the Middle East.
The board added the offer "still fundamentally undervalues easyJet and its prospects".
The value of easyJet's shares dipped by more than a quarter to a mid-May low of £3.40 following the US and Israel's assault on Iran, which started on 28 February.
However, this has all-but recovered to within a few pence of a 2026 high of £5.22. As of 8.45am on Monday (22 June), easyJet's shares were worth £5.19.
Castlelake said its latest offer representing "compelling value" for shareholders and accused easyJet's board of being "unwilling to engage meaningfully" with its interest.
"Castlelake's ambition is to support easyJet as a stronger, more resilient European airline under European control, respecting easyJet's valuable airline assets and continuing to sustain its network," it added.
The offer would see Castlelake take a 49% stake in easyJet, with the remaining 51% due to be held by EU nationals to satisfy European Union ownership stipulations.
EasyJet, by contrast, said the offer was based on "Middle East conflict-affected share prices, short-term earnings and analyst reports" and failed to take account of its medium-term prospects, strong balance sheet and capital structure.
"Accordingly, the board believes the third proposal represents an opportunistic attempt to acquire easyJet 'on the cheap' and that it is therefore not in the best interests of easyJet shareholders."
Last month, easyJet recorded a deeper first half (six months to the end of March 2025) headline loss before tax of £552 million – an increase from £394 million compared with the same period a year earlier.
This was partially offset by the continued strong performance of easyJet holidays, which delivered a first half headline profit of £61 million following a more than £100 million increase in revenue from £400 million to £518 million off the back of a 22% increase in passengers.
