Easyjet is introducing a 'cabin bag guarantee', which will reduce the size of hand luggage customers may be allowed to keep with them on the flight.
From 2 July, the airline cannot guarantee that if a customer brings hand luggage bigger than 50x40x20cm, it will not be placed in the hold.
Easyjet said there is no change in policy, and customers will not be charged for hand luggage bigger than the 'cabin bag guarantee'.
Currently, if a customer hasn't paid for checking in a bag, and they do this at the bag drop, they will incur a charge of £25. If they do this at the boarding gate, the fee rises to £40.
But the airline says anyone who has hand luggage bigger than its new restrictions but smaller than the maximum size of 56x45x25cm (and has to check-in the bag as a result) will not have to pay these charges.
Easyjet said it "carries around 40 more passengers" per flight than other airlines, and for this reason "overhead locker space onboard our aircraft is limited and so on busy flights we are not always able to fit everyone's cabin bag in the cabin".
A spokesman added: "There is no change to our bag policy which means customers can still bring a slightly bigger bag up to the maximum IATA size of 56x45x25cm, including handles and wheels, but on some of our busier flights their bag may have to go in the hold."
The budget airline said that it "does not charge passengers for bringing a cabin bag on board and there is no weight limit on cabin baggage".
Customers were emailed last week to inform them of the changes.