A Spanish court called Ryanair's budget airline's policy on charging luggage "excessive" after a passenger was fined for taking a handbag without a special ticket.
The passenger was forced to pay a fine of 20 euros (17 pounds) to bring her luggage of 10 kilograms.
Ryanair only allows small bags as carry-on luggage if they can be stored under the front seat.
The airline said it would not change its packaging policy.
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The passenger was traveling from Madrid to Brussels when she was ordered to bring her extra luggage.
The airline charges an additional fee for carrying more than one personal item on board. Larger bags may also require baggage allowance.
In its judgment, the Commercial Court ruled that the woman should be repaid with interest.
The luggage could easily be fitted into the cabin, the judge said.
He decided the policy was null and void and told Ryanair to "remove" it from his terms.
However, the compensation was ruled out because the judge did not consider that the case caused enough stress to the disgruntled passenger.
The airline said it would not change its policy.
"This decision will not affect Ryanair's luggage policy, in the past or in the future, as it is an isolated case that misinterpreted our commercial freedom in determining the size of our household luggage," the statement said.
There is no appeal against the ruling.