M&S.com sales fell 8.1% during the three months ending 28 June, while overall general merchandise like-for-like sales fell 1.5%.
Marc Bolland, M&S’s chief executive, said that because of an "increased focus on margin [M&S was] less promotional, both online and in stores"; although the retailer had said in April that it was planning a marketing campaign for May to promote the new site after it presented an in-depth overview of the overhaul to analysts.
Commenting on today’s results, Bolland said that the new website, which was previously powered via a now-terminated partnership with Amazon, has been "technically resilient" and garnered "positive customer feedback on [its] enhanced fashion and style content".
He added: "As previously indicated, we are now focused on optimising the website commercially, with updates made on a regular basis. We have recently seen a gradual improvement in sales performance, despite a lower level of promotional activity. We expect to return to growth ahead of our peak trading period."
Offsetting the lacklustre performance across general merchandise, like-for-like food sales were up 1.7%, while international sales grew 4.7%.
But James McGregor, director of retail consultants Retail Remedy, questioned the apportioning of blame for ailing sales to the new website. He said: "This is the last quarter that the bedding-in of the new website can be used as an excuse for the poor performance of clothing.
"Alongside clothing, the ongoing performance of food is starting to look tragicomic. Clothing at M&S is still very much on the rack and we will need to see strong numbers on a consistent basis to be convinced otherwise.
"The problem M&S is facing is the biggest problem of all for a retailer – identity. It still seems unsure who its target market is; the young or the old? M&S wants to be an international, multi-channel brand, but will it ever achieve this while its identity problems continue?"