Ryanair has warned that its policy regarding carry-on baggage "may be reviewed" if customers continue to bring large bags on board.
The low-cost airline said some travellers are flying with bags that exceed the maximum dimensions allowed.
This can cause disruption as they struggle to fit them into the aircraft's overhead lockers.
Many passengers refuse to pay for checking in their luggage as fees can reach £60 for each 20kg bag when booked online.
Ryanair said in a statement that its flights are 95% full this summer so passengers "must comply" with baggage rules.
The airline's chief marketing officer Kenny Jacobs warned that the airline's hand luggage allowance of one normal cabin bag and one small bag, such as a handbag or laptop, could be reduced unless passengers change their behaviour.
He said: "We've noticed some customers are bringing larger than permitted bags on board, which can cause delays, and our policy may be reviewed should this practice continue.
"As we enter the peak summer period with many full flights, we urge customers to ensure that they travel with less carry-on bags where possible.
"Our aircraft can only carry 90 larger carry-on bags and our gate agents will rigorously enforce our carry-on policy to avoid flight delays and ensure an enjoyable travel experience for all customers.
"Any customers who wish to carry larger baggage are advised to purchase a checked-in bag."
Ryanair today launched a huge winter seat sale, with over 250,000 seats on more than 900 routes on sale from €9.99, for travel in October and November.