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The Amazon Prime 30-day free trial advertisement was "likely to mislead" customers, the Advertising Standards Authority (ASA) says.

Today's ruling means the advert must not appear again in "its current form".

Six customers complained to the ASA over the Amazon trial, saying that it was not clear "that a paid subscription would automatically start" if not cancelled during the free trial.

The ASA ruling does not influence current payments or refunds.

As well as upholding that complaint, the ASA also ruled that the price of the subscription was not made obvious enough, as it was not in the original advert.

A 12-month subscription to Amazon Prime cost £79.

Last month many users took to social media to complain about the service.

Reports suggested that Amazon Prime added 10 million new subscribers in the last three months of 2014 alone.

It was also claimed that Amazon Prime members now represent nearly half of all Amazon customers.

It's not known how many users there are in the UK, but Amazon told Newsbeat it's "millions".

The complaints to the ASA centred around a letter that was sent to customers with Amazon accounts, which included a plastic card, directing people to Amazon UK.

In their defence, Amazon UK's parent company, Amazon Europe Core Sarl, pointed to some "small text" at the bottom of the letter in the offer terms, which stated: "Paid subscription starts automatically after free trial unless cancelled."

They also said: "During the online registration process customers were again made aware that they would be charged a fee."

The ASA said that the small print was not enough to warn consumers that the trial would end in a paid subscription if not cancelled in time.

It was also ruled that the price of the subscription to Amazon Prime was "material information" that should have appeared in the advert.

Amazon have been told by the ASA that in future the automatic start of the paid subscription must appear in the main body of the advert. Customers should also be told about the cost.

If you want to avoid your free trial being extended to a paid service go to Your Account on Amazon and adjust your membership settings within 30 days of signing up.

You can cancel your membership in Your Account at any time.

Full refunds are only given if you've not used any of the Prime benefits.

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