The budget airline's shares have rallied 43% in the last three months, helping them secure a place again after being demoted in June.
Miner Fresnillo and insurer Hiscox will be relegated.
The demotions were widely expected based on the recent closing prices of the stocks that saw them ranked at 111 or lower.
The following stocks will enter or leave the midcap FTSE 250:
ENTERING FTSE 250:
C&C Group
LEAVING FTSE 250:
NB Global Floating Rate Income Fund (GBP)
(Reporting by Joice Alves, Editing by Rosalba O'Brien)