Takeover target easyJet will return to London's premier FTSE 100 index, with British oil and gas company Ithaca Energy entering the top flight for the first time, according to FTSE Russell.

They will replace sports-betting and gambling group Entain and housebuilder Persimmon, FTSE Russell said.

Budget airline easyJet left the index on March 23 after a two-year stint in the top flight. Its shares closed at 673.20 pence on Wednesday, giving the company a market value of 5.11 billion pounds ($6.89 billion). They are currently up nearly 47% over the past three months. Last month, Apollo Global Management agreed to buy the carrier for the equivalent of around $7.7 billion, prevailing in a weekslong bidding war for the airline.

Aberdeen, Scotland-headquartered Ithaca Energy, which operates in the North Sea, has seen its share price climb 21% over the past three months. It has a market capitalization of 4.74 billion pounds based on its closing share price of 286 pence on Wednesday.

Shares in Entain, which houses British brands Ladbrokes and Coral, are down 6.5% over the past three months. It last joined the index in June 2020.

Persimmon's shares, which last joined the index in January 2024, are down 16% over the year to date and up 6.45% over the past three months.

The changes will be effective from the start of business on Sept. 21.

The FTSE 100 is a share index of the 100 most highly capitalized companies listed on the London Stock Exchange. Any company that falls to 111th and below is automatically ejected from the top-flight index, while any firm that rises to 90 or above is automatically promoted.

FTSE Russell is wholly owned by London Stock Exchange Group.