EasyJet has rejected a takeover offer worth £4.74bn from US investment firm Castlelake, describing the bid approach as "highly opportunistic".

Castlelake said it had made three approaches to EasyJet this month, all of which had been rejected, but it had now made details of its latest offer public to allow shareholders to assess the proposal.

The US fund, which already owns a stake of about 2.14% in EasyJet through the funds it manages, has until this Friday to make a firm offer or walk away.

Under Castlelake's latest offer, the airline's shareholders would receive 625p per share, a 24% premium to last Friday's closing price.

"Following the rejection of three proposals by the EasyJet Board, and given its unwillingness to engage meaningfully, Castlelake is announcing this Third Proposal to enable EasyJet shareholders to consider its merits," Castlelake wrote.

It said its latest bid "offers compelling value" to EasyJet's shareholders.

"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," the US firm said.

European Union regulations stipulate that EasyJet must be majority-owned by EU citizens. Castlelake said it had proposed an ownership structure which was a "deliverable solution to ensure compliance with all applicable regulatory requirements."