Mobile default ads, where static images are served in place of rich media when Flash-based ad formats aren’t supported, resulted in 5.35bn wasted rich media impressions on mobile devices in the first quarter of 2015 alone.

The figures come from Sizmek’s Mobile Index, which examines trends and growth in mobile inventory, and also found that HTML5 ads, which typically perform equally well on mobile and desktop, outperformed Flash ads by 400 per cent in terms of interaction rate, thanks largely to this advantage.

Rich media ads that relied on Flash served their default image on mobiles an astounding 98 per cent of the time, while those in HTML5 only defaulted for 8.3 per cent of impressions, highlighting the considerable difference between the two options.

However, the relatively slow uptake of HTML5 has meant that 12 per cent of advertisers have never successfully served a rich media ad to a mobile device, considerably impacting click-through rates for publishers, advertisers and brands.

“Advertisers are using more rich media on mobile to reach potential customers, but they’re wasting a lot of opportunities,” said Andy Kahl, director of research at Sizmek. “By converting rich media ads to HTML5, brand advertisers can ensure their ads are seen as intended, thus increasing return on their ad spend.”

Sizmek also announced that its own Ad Builder for HTML5 would now feature video autoplay in mobile, enabling advertisers to produce soundless teasers for mobile video ads that play immediately, without any user interaction needed.

“While video on mobile has increased around 300 per cent over the last six months alone, marketers have been missing out on a huge opportunity to leverage it more effectively to connect with mobile audiences,” said Alex White, vice president of product marketing at Sizmek. “Autoplay helps solve the problem by enticing mobile users to engage with a video, but in a respectful way, empowering advertisers and audiences alike.”