The UK's advertising watchdog will reportedly tell Amazon this week that it can no longer make "misleading claims" about its Prime service guaranteeing next-day delivery.

The ruling from the Advertising Standards Authority (ASA), obtained by The Times but due to be officially published this week, comes after 270 customers reported not receiving their delivery by the following day in the run-up to Christmas.

"Because consumers were likely to understand that, so long as they did not order too late or for Sunday delivery, all Prime labelled items would be available for delivery the next day with the One Day Delivery option,” the ASA is expected to say.

"When a significant proportion of Prime labelled items were not available for delivery by the subsequent day with One Day Delivery, we concluded that the ad was misleading."

The ASA will rule that Amazon's ads "must not appear again" in their current form.

An Amazon spokesperson stressed that the "overwhelming majority" of one-day delivery orders are delivered when promised.

"The expected delivery date is shown before an order is placed and throughout the shopping journey and we work relentlessly to meet this date," the spokesperson added.

"A small proportion of orders missed the delivery promise last year during a period of extreme weather that impacted all carriers across the UK, and we provided support to impacted customers at the time.”

The regulator had previously ruled against Amazon in 2016 over its free delivery claims, saying its ads "did not make sufficiently clear which items were eligible for free delivery, and under what terms, and that they were therefore misleading".