During the unveiling of M&S' two, shorter Christmas ads for 2015, Patrick Bousquet-Chavanne, the executive director of marketing and international, warned that all marketers should be worried by ad blocking.
He told Marketing: "It is a concern, I do not think there is any marketeer that would not be concerned by it."
Bousquet-Chavanne pointed to earlier technologies that allowed consumers to skip TV ads en masse.
He said: "We were also all concerned when video-on-demand came out. Those are the transformations of the ways consumers are viewing and consuming media right now, and we have to watch it very closely to make sure marketing spend remains very efficient."
But, he added, M&S's use of analytics gave him confidence that the retailer wouldn't waste money targeting consumers who aren't interested in its ads.
The retailer has put a quarter of its - undisclosed - Christmas campaign spend into digital, and will be chopping the two ads into a series of shorter, attention-grabbing spots intended to work well on mobile.
It has described its Christmas campaigns as one made up of a series of "moments" that mark the run-up to Christmas, with one ad showing preparation like getting ready for the party season, wrapping presents on Christmas Eve and the feast itself.
The concept of "moments" is increasingly common in mobile marketing language, playing to the idea that consumers are constantly scrolling through their social feeds. This gives marketers a finite amount of time to capture their attention, resulting in shorter, highly focused creative.
But as marketers finally get to grips with strong mobile creative, consumers are finding ways to circumvent advertising.
Ad blocking has, up until now, largely been confined to the desktop.
According to an annual report from PageFair and Adobe, use of ad blockers in the UK rose 82% in the last year, with 12 million monthly users on average. That's mostly on desktop, with mobile still negligible.
But moves by Apple to enable the technology on iPhones for the first time mean the trend could go mainstream on mobile.
Still, M&S boasts impressive numbers on its owned media properties, suggesting its campaign will have strong reach regardless of ad blockers.
Bousquet-Chavanne said M&S has seen a considerable increase in sign-ups to its social channels, boasting 4.5 million fans across its accounts, though some of these are likely to be replicated.
It also has two million customers signed up to Sparks, its loyalty programme, who will be drip-fed the campaign first.