Under Ryanair's increasingly strict cabin baggage policy, which was last updated in November 2018, passengers can only take one bag measuring up to 40cm x 20cm x 25cm into the cabin with them, unless they pay extra.
The investigation, by MoneySavingExpert.com, found that Debenhams was selling a "cabin suitcase" measuring 55cm x 39cm x 20cm with a description which said "fits all major airlines including Easyjet, British Airways and Ryanair".
The bag was listed by a concession holder, Tripp Luggage, which apologised for the "confusion". Tripp Luggage said: "Unfortunately there are some cabin pieces on the website that have not been updated to ‘Ryanair Priority’ following the change to Ryanair’s cabin restrictions. The description on these pieces is being updated as a matter of urgency and will be live on the Debenhams website on 29 August.
Argos had a section on its site called 'cabin luggage size guidelines', which stated Ryanair's dimensions as 55cm x 40cm x 20cm.
Argos has now changed its website, clarifying the correct hand luggage rules and listing bags that would be the right size for priority and non-priority Ryanair passengers separately.
The report found that bags sold through third parties by online retailers such as eBay and Amazon, were also advertised as suitable for Ryanair, but fell foul of the rules.
A spokesman for eBay said: "Ebay policy states that sellers must always describe products accurately and they can face action for not doing so. Our Money Back Guarantee means that if an item is delivered not as it was described, the customer will be entitled to a full refund from the seller.”
Amazon said it would be removing all three specific listings we flagged to it and asking the third-party seller to update the product descriptions so that they are accurate.
Steve Nowottny, news and features editor at MoneySavingExpert.com, said: "Ryanair's baggage rules are super-strict, and given they changed twice last year, passengers can unsurprisingly get caught out. But retailers need to do what they can to help and make sure they're really clear when selling luggage.
"If a bag is marketed as being suitable for Ryanair, many will assume that means you can fly with it for free. If that's not the case, shoppers should be clearly warned.
"It's good to see a number of stores making changes to their sites off the back of our investigation, but if you're a passenger looking for a new cabin bag, the message is clear. Don't assume you'll be able to fly with a bag for free based on the marketing spiel alone. Instead, check the exact measurements of any bag you plan to buy, and your airline's allowances."