Consumer experts are warning passenger to beware of Ryanair's strict rules on luggage to avoid paying hundreds of pounds in fines.

Travellers flying on Ryanair are all too aware of the airline's love of ensuring bags meet exact dimensions. However, one family reportedly got a nasty shock when they were hit with a fee of £400 after not checking the airline's rules on baggage.

It comes as consumer experts at Which? gave a number of rules in order to save as much money as possible flying with the no-frills airline. Ryanair, for its part, has said that people should be avoiding paying high fees for oversized luggage by sticking to the dimensions set out in their luggage rules, slamming the Which? report as "false" and "inaccurate".

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In the Which? report, their travel insiders set out a series of guidelines in order to avoid paying high costs with Ryanair, headed by CEO Michael O'Leary. They insisted the number one rule to follow is to make sure the bag you are bringing really does fit in the holder. Ryanair are still extremely strict on the exact dimensions of the bag being brought, and if your bags don't fit in the sizer at the airport, you could end up being charged for oversized luggage.

One person claimed via Which? Travel's Facebook page that they were slapped with a £400 charge for a family of four. The family had been travelling on return flights, but had slipped up after not checking Ryanair's rules on how luggage should be brought on board.

In a statement responding to the claims, Ryanair said: "People should avoid Ryanair's high fees for oversized bags. They can do so by simply travelling with bags that meet our agreed dimensions. Our sizers are bigger than our permitted dimensions, so if that the bag fits in the sizer, it gets on, if it doesn't, it gets charged."

Ryanair is known for its strict hand luggage rules, which are set out for passengers on its website.

If you are paying a "Basic" fare without priority boarding, the budget carrier only permits one bag that is meant to fit underneath the seat in front of you. The current dimensions that are accepted for this under-the-seat bag are 40cm x 30cm x 20cm (aka a small rucksack or handbag).

If you want a second bag, you can opt for its Priority & 2 Cabin Bags bookings, which allows you to not only have the under-the-seat bag, but you can bring a second case as long as it's no larger than 55cm x 40cm x 20cm. These measurements also include the wheels and handles of the bag, so if the bag would fit in the hold without the wheels but just breaches the rules with them, the bag is deemed as oversized.

For passengers who fall foul of the rules, there are charges ranging from £46 to £60 for an oversized non-priority bag, and this can go up to £75 for an oversized cabin bag. However, it is important to note that the cost can vary depending the route.

It's worth noting that these are the rules for cabin bags - if you want checked luggage, you can add these to your booking with 10kg, 20kg and 23kg bag options.

Have you been caught out by an airline's luggage rules? Email us at webtravel@reachplc.com