M&S bosses have been warned about their future advertising after a claim that its Christmas dinner was "Â£10 cheaper" was found to be "misleading."
The ad also claimed that M&S was "the only supermarket where Christmas dinner cost less than last year."
But it turned out that M&S had replaced a fresh turkey with a frozen one and the Christmas Cake submitted for the survey was of lower quality than the one used in 2016.
Watchdogs found the ads, published the week before Christmas, breached rules regarding "substantiation", "qualification", "comparisons with identifiable competitors" and "price comparisons" as well as "misleading advertising."
An investigation was triggered when two complaints were received by the Advertising Standards Authority (ASA) about the ad that stated: 'We're Â£10 cheaper than 2016 as reported by Good Housekeeping Institute. M&S. The only supermarket where Christmas dinner cost less than last year."
At the bottom of the page was small print that stated, 'Based on Good Housekeeping Survey of the top 10 supermarkets."
It added: "Based on an independent survey by Good Housekeeping of comparable products for a Christmas dinner for 8 people with Frozen Stock Basted Whole Turkey (2017) in place of Fresh Whole British Turkey (2016) and a Classic Christmas Cake (2017) in place of a Collection Christmas Cake (2016)."
Two people challenged whether the claim 'We're Â£10 cheaper than 2016' was misleading, because the comparison was based on the 2017 M&S Christmas dinner replacing two items from the 2016 M&S dinner with cheaper alternatives.
They also challenged whether the comparative claim 'the only supermarket where Christmas dinner cost less than last year' was misleading and could be substantiated.
The ASA upheld the complaints and banned the ad from appearing again.
M&S said the intention of the Good Housekeeping Institute survey on which the claim was based was to highlight what shoppers could pay at different supermarkets for a typical Christmas dinner based on a specific basket of items and how the price of that basket then compared with the supermarkets' 2016 equivalents.
M&;S bosses have been warned about their future advertising after a claim that its Christmas dinner was "Â£10 cheaper" was found to be "misleading."
The retailer stated that the results of the survey indicated that the price of an average basket for Christmas dinner at M&S was less in 2017 than it had been in 2016.
In terms of the items and data submitted in 2017 compared with 2016, there were two differences that were not identified by the survey, and were not made available on their website for consumers to view.
The firm stated that the first difference was that M&S submitted a frozen turkey as part of their 2017 Christmas dinner, whereas in 2016 it had been a fresh turkey.
They claimed it was because they did not have a frozen turkey available to submit in 2016.
M&S said the survey specifically required a frozen turkey in 2017 and therefore, there was no option but to compare the 2017 frozen turkey with the 2016 fresh turkey.
They said the second difference was that in 2017, they were able to offer a 'Classic Christmas Cake' at a better value price than the 'Collection Christmas Cake' they sold in 2016.
M&S "strongly believed" that the products were comparable and taken together with the details made available in the ad's small print, they considered the ad was not misleading and had in fact provided more information than shoppers could have obtained elsewhere.
But an ASA spokesman said: "We considered that consumers would interpret the claim 'We're Â£10 cheaper than 2016' to mean that they could purchase the same type and quality of traditional Christmas food items that M&S offered in 2016, but that they would be Â£10 cheaper in comparison.
"We noted that the claim featured asterisks that linked to the small print at the bottom of the ad.
"That meant that the M&S 2017 Christmas dinner deal was different from the 2016 Christmas dinner offer, specifically that it had replaced a fresh turkey with a frozen one and a high-end cake with a cheaper alternative.
"Whilst we agreed that these substitutions were of the same type of product they were not of the same quality.
"Although the qualification explained the differences, we considered it was not sufficient to override the overall impression of the ad."
He added: "M&S' 2017 Christmas dinner deal was different from their 2016 Christmas dinner offer, because it had replaced a fresh turkey with a frozen one and a high-end cake with a cheaper alternative.
"Because consumers were likely to understand that the comparisons were like-for like, when M&S had compared their Christmas dinner items with cheaper alternatives from 2016, but had not provided evidence that the other supermarkets had done so too, we concluded that the comparative claim 'The only supermarket where Christmas dinner cost less than last year' was misleading.
"We told M&S that their future advertising for such offers must not suggest that they were offering items of the same quality as in previous years when that was not the case.
"Furthermore, they must ensure that their advertising did not suggest that an offer was a like-for-like comparison with their competitors' unless that was the case and could be supported with adequate evidence."