Gap year travel insurance customers may be among those who face paying extra charges on top of standard airfares and taxes.
According to consumer specialist Which? Holiday, many operators are using charges to recoup some of the losses made from reduced passenger numbers.
A spokesperson for the publication pointed to announcements made earlier this year by British Airways, Air France and Virgin Atlantic, confirming the firms were all losing revenue.
Charging passengers extra for their luggage is one way British Airways is attempting to make back some of the money lost, he added.
Air France now also requires travellers departing from Europe to pay a 50 euro (£46) fee to check in a second bag.
The spokesman advised customers to consider carefully what the full cost of a flight will be before handing over any money.
"While you are booking, check you are not agreeing to pay extra charges when you need to when booking on the internet," he said.
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