Consumers are being "fooled" into paying more than they need to for flights and holidays in misleading time-limited deals, a watchdog has found.
Which? Travel found some popular holiday companies may be breaking the law with adverts misleadingly portraying a deal as a time-limited bargain.
The consumer group tracked time-limited deals over three weeks in July and August and found the same offer was available for the same price or cheaper in 16 out of 30 cases after the deal had ended.
Holiday companies Kuoni and Inghams told Which? they had reviewed how they promote offers as a result of the findings, which the watchdog has shared with Trading Standards and the Advertising Standards Authority.
Advertising such deals could breach consumer regulations if they are found to be misleading and are likely to lead to the average person going ahead with a transaction they would not otherwise have made.
In October, Sandals offered a seven-night all inclusive break to Jamaica from £1,465 per person as part of its Summer 2018 Mega Sale, but the price dropped by £50 per person the day after the sale ended, Which? found.
Its Halloween promotion followed immediately after, with another seven days added to the same deal.
Lastminute.com's "flash sale" offered a night's stay at the Hotel R Kipling in Paris with flights from London discounted to £139 on bookings made by August 23.
But the following day the same night's stay at the same hotel fell to £126 and was still available a week later.
The Virgin Holiday Sale offered seven nights at Florida's Coco Key Hotel from £792 per person if booked by August 17, but the same dates dropped to £677 per person the day after the sale.
Meanwhile, two nights at Paris' Ares Eiffel hotel were available for £404 through Expedia's "flash sale" but a fortnight later a new 40% off promotion was running and the same stay was available for £382.
Inghams Italy's website offered discounted trips to Capri, Milan and Puglia until August 4 but the expiry date was pushed back twice, meaning the same prices were still available a month later.
Which? Travel editor Rory Boland said: "Our investigation shows that most of the time-limited deals we looked at are nothing of the sort.
"The tight deadlines, emotive language and countdown clocks are all designed to rush us into making a decision, but it might not be the right one.
"Don't be fooled - compare prices with other holiday companies and travel agents to check savings are genuine."
A spokeswoman for Unique Caribbean Holidays Ltd, the UK tour operator for Sandals Resorts, told Which? the company did not intentionally pressure sell or create false book-by dates, and that all of the Sandals and Beaches Resorts' fully protected packages were fairly promoted to the customer.
Virgin Holidays said it would never intentionally advertise anything misleading.
The company added: "We're always looking to secure the best possible value for our customers, and should we be able to obtain better offers from our suppliers, these savings will be passed on to benefit the customer."
Lastminute.com said it aimed to comply with all advertising regulations, adding: "It is not, and never has been, our intention to mislead our customers."
Expedia thanked Which? for raising the issues it found in the investigation and said it was going to investigate them.
A Kuoni spokesman said: "It's never our intention to mislead customers.
"These offers should have been promoted as special offers rather than a sale, and we have ensured processes are in place to clarify this going forward."
Inghams told Which? the webpage in question no longer existed and promised to "review all procedures".