An offer of a “free trial” of Amazon’s Prime delivery service has been banned in Britain after a watchdog found the online retail giant misled consumers about subscription fees.

The direct mailing sent to consumers with Amazon accounts included a plastic card stating "30-day free trial", information about the associated Amazon Prime Instant Video and a letter reading: "Dear (recipient's name), I'm sending you this letter because I want you to know that you are eligible for a free trial of Amazon Prime ... Start your 30-day free trial today and watch as much as you want ... That's all there is to it ..."

Small text at the bottom of the letter said: “Paid subscription starts automatically after free trial unless cancelled.”

Six people complained that the ad was not sufficiently clear that a paid subscription would start automatically if not cancelled during the trial and did not state the cost of the subscription.

Amazon Europe said the ad repeatedly said the “free” element of the trial was time-limited and, on all but one of the occasions on which the word “free” was used, it was preceded by “30-day”.

They said it was common practice for free trials for services to convert to a paid subscription unless cancelled, and that consumers would understand and expect that they had to cancel within the free period if they did not want to start a paid subscription.

The Advertising Standards Authority (ASA) said a paid subscription starting automatically at the end of the trial was a commitment and a significant condition of the "free" offer, and should therefore be made clear to consumers.

It said: “We did not consider that it was sufficient to include the information about the automatic paid subscription in the small print of the ad only and therefore did not consider that that information was sufficiently prominent to make clear the extent of the commitment consumers must make to take advantage of the offer.

“We concluded the ad was likely to mislead.”

The ASA said the ad was also misleading for not saying that a subscription to Amazon Prime cost £79 a year or that the charge for Amazon Prime Instant Video was £5.99 a month.

It said: “We considered that the amount of the subscription fee was material information and a condition of the offer of which consumers should be made aware.

“We did not consider it was sufficient to set out that information in the subsequent online registration process. Therefore, because the subscription fee was not detailed in the ad, we concluded that the ad was likely to mislead.”

The ASA ruled that the ad must not appear again in its current form.