Holiday firms which advertise time-limited offers may be misleading consumers, according to the consumer group, Which?
Many tour operators promote money-off deals, providing travellers book by a certain date.
But when Which? studied such offers, it found that half the holidays advertised were the same price -or even cheaper – after the offer expired.
The firms involved have all denied misleading their customers.
However two operators – Kuoni and Inghams – told Which? they had reviewed the way they promote their holidays as a result of the findings.
Which? looked at 30 examples over a three-week period in July and August 2017.
In 16 of those cases, it found prices were lower, or the same, after the deal was over.
Which? claimed that in some cases the promotions could be in breach of consumer law.
In one example, Virgin Holidays advertised seven nights in a Florida hotel for £792 per person, if it was booked by 17 August.
But according to Which? the day after the sale the price dropped to £677 per person – a saving of £230 for two people sharing.
In October the Caribbean resort Sandals advertised an all-inclusive break to Jamaica from £1,465 per person.
Yet the day after the "mega sale" ended, the price dropped by £50 a person.
Virgin Holidays told Which? that it would never intentionally advertise anything misleading.
It 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."
Unique Caribbean Holidays – which represents Sandals in the UK – said the company does not intentionally "pressure sell", or create false "book by" dates.
It said all its packages were fairly promoted to the consumer.
However Which? is advising holidaymakers to take their time before deciding to make a booking.
"Our investigation shows that most of the time-limited deals we looked at are nothing of the sort," said Rory Boland, Which? travel editor.
"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."
The Association of British Travel Agents (ABTA) said that most of its members follow its code of conduct, which stipulates that promotions should not be misleading.
"The Which? findings relate to a small number of instances, and we do not believe they are reflective of wider travel industry practice," an ABTA spokesperson said.
"The number of complaints we receive each year about holiday pricing is very low – indicating that the majority of price advertising is trustworthy."