EasyJet has called a potential £3bn bid by a US investment group “highly opportunistic”, as shares in the budget airline shot up to their highest level in three months on the takeover interest.

The US private credit firm Castlelake said on Friday it was considering a takeover offer for the airline. On Monday, it said it had already bought a 2.14% stake in the business and its offer would value easyJet at least at 403p a share, or about £3bn overall.

However, easyJet hit out at its potential buyer, saying it was “highly opportunistic timing” as its share price was “temporarily depressed due to the current situation in the Middle East and its impact on customer confidence and jet fuel prices”.

Before news of takeover interest emerged, shares in easyJet had lost about a fifth of their value since the start of the year.

However, the company said its board was confident in its strategy given its cash position and profit outlook.

Shares in easyJet shot up by as much as 12% in early trading on Monday, reaching 444.7p – well above the minimum level of a potential offer by Castlelake, and their highest level since 2 March, valuing the company at about £3.4bn. The jump later eased, with shares up about 10%.

Under City takeover rules, Castlelake, which is headquartered in Minneapolis and manages $36bn (£27bn) in assets, has until 5pm on 26 June to announce whether intends to make an offer for easyJet.

EasyJet said it would “consider any proposal, should one be made” but that there were “considerable regulatory, financial and other execution challenges associated with a potential takeover”.

Under EU rules, European airlines must be majority owned by investors within the region. Ruairi Cullinane, an analyst at the broker RBC Capital Markets, said the rules “could, at the very least, complicate a takeover of easyJet by Castlelake, if acting alone”.

It is not the first time that the FTSE 100 airline has attracted potential buyers. In October, reports emerged that the Swiss-headquartered shipping company MSC was considering a takeover of the business. In 2021, the company also rejected an approach from its rival Wizz Air.

EasyJet, which is headquartered in Luton and employs more than 16,000 people around the world, is one of Europe’s three biggest budget airlines, behind Ryanair, with Wizz Air in third place.

The business was founded by the billionaire Stelios Haji-Ioannou. He remains the biggest single shareholder in the company, with a stake of about 15%. Haji-Ioannou declined to comment on Monday.

Castlelake already has a strong presence in the airline industry, having provided loans to the Scandinavian airline SAS, as well as Virgin Atlantic Airways.

Susannah Streeter, the chief investment strategist at Wealth Club, said: “Castlelake clearly believes the market may be underestimating easyJet’s longer-term earnings potential and the resilience of its network.”

A takeover of easyJet would mark another significant loss for London’s struggling stock market, which has suffered a series of high-profile exits in recent years, including the construction equipment rental company Ashtead, the gambling group Flutter Entertainment and the building materials provider CRH.

Streeter added: “This is fresh evidence that the British markets are increasingly becoming a hunting ground for sophisticated institutional investors, with UK-listed stocks continuing to trade at lower valuations than other markets.”