Amazon is to be told to stop claiming its Prime service guarantees next-day delivery by the UK advertising regulator after customers complained it was failing to provide their goods on time in the run-up to Christmas, it has been reported.

The Advertising Standards Authority (ASA) is expected to rule that the firm’s claims to be able to operate an “unlimited one-day delivery” service are misleading in the case of some items.

“[A] significant proportion of Prime-labelled items were not available for delivery the next day … because consumers were likely to understand that, so long as they did not order too late, all Prime items would be available for delivery the next day … we concluded that the ad was misleading,” the ASA’s ruling will say, according to the Times.

Amazon offers the service to its Prime customers, who pay a £7.99 monthly subscription, but many people said it failed to live up to its promises last Christmas.

It is not the first time the online retailer has been criticised by the regulator. In 2016, the ASA ruled against Amazon 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”.

The firm had not made clear that the claim did not cover items it was not itself responsible for dispatching.

Amazon has also come under attack over its tax affairs and, last week, it was revealed that it had almost halved its UK corporation tax bill while tripling its profits.

On Friday, the chancellor hinted that he could change the tax system for online sales as the scale of the difficulties experienced by the high street store, House of Fraser, became apparent.

“We want to ensure that the high street remains resilient, and we also make sure that taxation is fair between business doing business the traditional way, and those doing business online. We may have to look at temporary tax measures to rebalance the playing field until we can get international agreements sorted out,” Philip Hammond said.

A spokesman for the ASA said: “We have been formally investigating Amazon’s ‘one-day delivery’ ad claims and will publish our findings in full soon.”

Amazon did not respond to the Guardian’s request for comment but the company told the Times its Prime service still offered “fantastic benefits”.

A spokesperson said: “The expected delivery date is shown before an order is placed and throughout the shopping journey and we work relentlessly to meet this date.

“A small proportion of orders missed the delivery promise last year during a period of extreme weather that impacted all carriers across the UK.”