Online travel agents are today accused of luring holidaymakers in with deals that are “too good to be true” before hiking prices.

In some cases, prices have rocketed during the booking process, a Which? Travel investigation found.

In others, customers complained of being called the day after a booking only to be told that, if they don’t pay more money, their booking will be cancelled, according to the consumer watchdog.

The probe highlighted passenger Kam Hughes who booked a flight to Canada after her mother fell ill in Vancouver. The cheapest she could find was with Checknfly but the day after booking she was    told that if she didn’t pay another £79 her flight would be cancelled.

She refused and booked a flight direct with British Airways instead.

Checknfly told Which? Travel it wasn’t its fault and that the price had increased by £160 while she was making her booking.

“We were ready to bear half the loss and requested that Mrs Hughes pay £79 in order to secure the flights,” it said. “When she refused we processed the full refund back onto the card she paid from.”

Another holidaymaker, John-Michael Clow, found a flight to India with Gotogate that was almost £200 cheaper than anywhere else.

But when he got to the airport, Thomas Cook Airlines told him that his ticket was not valid. Thomas Cook had already informed Gotogate that there was a problem with the ticket at the time of booking, but Gotogate had failed to inform Clow.

He called Gotogate, but the firm insisted that it hadn’t done anything wrong and wouldn’t help. Even when he finally got a refund from Gotogate, the company deducted £17 as a ‘refund fee’.

Only after Which? Travel got in touch did Gotogate admit that it had made a mistake “due to human error”. It refunded the cost of booking a second flight and added compensation of just over £300.

“We have many customers and, of course, make mistakes now and then,’ the company said. “When that happens, we try to correct the mistake as soon as possible and compensate if necessary.”

Clow had found Gotogate through Skyscanner. The price comparison site said that it takes responsibility for the behaviour of agents that it lists.

“We log complaints, which then form part of the provider’s quality rating,” it said. “We will take action against providers who breach our strict thresholds, or have consistent patterns in negative feedback.”

One reason some OTAs are able to sell ultra-cheap tickets is because they sell unnecessary add-ons as well, according to Which? Travel.

Gotogate has a list of 11 extras that customers are offered before they can finally pay. These include £19.90 for a ‘platinum support’ package to guarantee it will respond to customer queries quickly. It also offers a bag tracking service through a company called Blue Ribbon – £9 to guarantee that if a customer’s bag goes missing for more than 96 hours they’ll be awarded $1,000 (£755).

But the same service is available direct from Blue Ribbon for $5 (£3.80). One of the worst value products is the £39 it was found to be charging for the US visa-waiver scheme Esta. Bought directly from the US government, it costs just $14 (£11).

“Our business is based on offering our customers the cheapest available air fare,’ Gotogate told Which? Travel. “We then offer several add-on products so they can customise their trip.”

Other agents appear to offer cheaper fares than the airlines but make up the difference by charging a supplement for taking luggage. One flight to New York with Norwegian cost £362 return with eDreams and £369 with the airline itself. However, Norwegian charges £35 each way for luggage, while eDreams adds a surcharge of £13, bringing the price up to £48 each way.

Edreams admitted that it does have a surcharge on luggage and said: “The prices we advertise via aggregators are always achievable but, of course, might increase depending on the additional services added to a booking.”

Which? Travel recommended that holidaymakers research travel agents online before booking a cut-price deal, or book direct with one of the “better airlines” in its annual survey.

The magazine’s editor Rory Boland told The Times: “Comparison sites are the best place to start your flight search but if you are offered a too-good-to-be-true deal, do a bit of research on the travel agent selling the ticket.

“A Google search might be enough to put you off. Even reputable companies have complaints, but if disappointed customers are consistently making the same complaint on online forums and social media, that should be considered a red flag.

“If you’re unsure, book direct with the airline or a well-established travel agent.”

An Abta spokeswoman said: “Customers should also always be provided with their tickets after having paid in full, and we encourage people to check their paperwork once they’ve made a booking.

“We recommend that customers look for the Abta logo when booking their travel arrangements as the Abta code of conduct requires the highest standards of customer service of its members.”