Passengers who shop duty free could spend up to 150% more than if they were to buy the same products from online retailers, according to a study.
On average, duty free products were 17% more expensive than from online retailers, comparison site finder.com found.
The website compared the cost of duty free products with the cheapest prices online in mid-June.
Chocolate and sweets were on average 67% more expensive than at typical shopping outlets, with a bag of peanut M&Ms costing passengers £3.75 instead of the normal £2.
A 300g bar of Cadbury Oreo chocolate would normally cost £2, a full £3 cheaper than at the airport.
Suncare was the second most expensive category of goods at duty free, setting consumers back 37% more than elsewhere.
A bottle of Ultrasun Aftersun cost £6.31 at duty free, 63% higher than the cheapest online price.
Luxury goods such as sunglasses and watches are on average 29% more expensive duty free, finder.com said.
In one example, it found Ray-Ban 3025 Aviator sunglasses in gold with a brown gradient lens for £113.30 in airport duty free stores compared with £81 at an online retailer.
On average, wine and whisky were 8% and 9% more expensive respectively at duty free, while all other spirits were 5% cheaper than online.
Passengers could also find some good deals on makeup, with products 16% cheaper on average in duty free stores than online.
Finder.com chief executive Jon Ostler said: “Our research found that the average price for an item bought in duty free was 17% more than the cheapest available figures online, which included at least three products that were more than double that of the online price.
“However, it is worth noting that duty free can offer exclusives and products that aren’t readily available elsewhere so the best thing to do is to research before you buy.
“Smartphones have made this easier than ever and a quick check before you hand over the cash could stop you overspending by as much as £31 for bigger ticket items like Ray-Ban sunglasses.”