(Pocket-lint) - Amazon is apparently being investigated over claims that its next-day Prime delivery service doesn't actually deliver "unlimited one-day delivery". That's because - as you'll know if you're a Prime user - not all items do arrive the next day. 

According to The Times, the UK Advertising Standards Authority (ASA) has been investigating 'late' parcels - primarily around Christmas delivery. The newspaper reports that up to 200 complaints have been lodged with the ASA, some of which centre around Prime items actually taking up to four days to arrive around Christmas.

Obviously, that is a busy period, but the ASA is expected to say that Amazon needs to better manage customer expectations and say whether or not it can deliver the next day. The recent Amazon Prime Day saw our Prime order take 48 hours to arrive, although Amazon was transparent about how long it would take. 

What it means in practice is that Amazon will probably have to clarify which items will or won't be delivered the next day, perhaps by removing the Prime branding from items that won't be with you in 24 hours. 

Amazon also fell foul of the ASA in April about the way it displays pricing and discounts - the rules around sale prices are strict for all retailers. One of the products concerned had been sold at a discounted price for a longer period, when Amazon had claimed it was a Prime Day discount. 

The news comes after the recent revelation that customers aren't really using Alexa to buy items, still preferring to see what they're buying. A report in The Information claims that only about 2 percent of those with Alexa devices have made a purchase with their voices so far this year. 

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Writing by Dan Grabham.