Undercover researchers found that scratchcard touts in popular Mediterranean resorts were luring British tourists into sales presentations with the promise of "star prizes".
To get the prizes, "winners" often have to attend lengthy presentations where they are sold membership of holiday clubs or timeshare trials.
Holiday Which? researchers, posing as an engaged couple, were approached by touts and enticed to two presentations, where they were bombarded with baffling information for up to five hours. The sales staff would not reveal the cost of membership until near the end of the presentations.
At both sessions the "couple" were pressured to sign up and pay money on the spot without the chance to think it over.
The magazine said people in the UK were also being targeted via a letter or a phone call telling them they had won a holiday prize.
The magazine added that it hoped legislation coming into force in April 2008, aiming to outlaw misleading and aggressive sales practices, would put an end to pressure-selling techniques at presentations.
Lorna Cowan, editor of the magazine, said: "At home or abroad, everyone likes to win a prize, especially a holiday.
"But there is a catch. You may have to attend a lengthy presentation where you are pressured into parting with thousands of pounds for something you might not even want.
"We welcome EU proposals which should prevent companies from demanding money on the spot for these types of holiday products.
"In the meantime we would advise holidaymakers to avoid scratchcard touts at all costs and not to buy into a holiday club."
About 140,000 consumers fell victim to foreign lotto ruses last year, losing a total of 260 million. The average loss was a massive 1,900 per victim, although the median loss was a more modest 42.
Among the best tips: never call numbers beginning 09 to claim a "prize" – it's a sure sign of a scam. If you've really won a prize, you shouldn't have to pay any administration fee.