The European Consumer Centre in Ireland is advising consumers to be cautious when signing up to free trial.
The Centre recorded an increase of 89 per cent in the number of free trial complaints received in 2014 with some consumers charged up to 1,200 euro by disreputable traders.
"We've seen a large increase in the number of people who've been caught out by what we consider free trial scams," said Grace Duffy of the ECC.
"Basically, what happens is the consumer signs up for a free trial for a certain period of something like a diet pill or skincare product, and pays a certain amount for postage and packaging.
"But what actually happens is they're inadvertently signed up to a subscription service which can then charge them - monthly - a very significant amount."
ECC Ireland said customers should research free trials extensively online, not relying only on a site selling the product - but to search for negative feedback from others, too.
If a customer finds themselves caught by a free trial scam, ECC Ireland said they some were able to arrange a partial refund with the trader - subject to a "costly administration fee" - or to get assistance from their bank or credit card provider.