The UK's burgeoning adtech market is helping fuel a boom in digital marketing, with double-digit growth expected to push spend above £15bn in 2019, according to a report by Barclays Corporate Banking.

But marketing leaders interviewed by Barclays also sounded a note of caution around relationships with the so-called Faangs – Facebook, Apple, Amazon, Netflix and Google – urging agencies to "retain their objectivity and discretion".

The bank's Adtech Ascendancy report was compiled after Barclays spoke to industry leaders – both chief executive and chief marketing officer-level clients as well as agency seniors – to gain insights into the prospects for ad agencies and how they might adapt to the changing marketplace.

Overall there was a healthy degree of optimism among interviewees, Barclays said, tempered by a recognition that a slowdown in the broader UK economy would have a knock-on effect for marketers.

Given the tech giants' command of massive revenues and their seemingly insatiable appetite for growth – Google and Facebook on their own account for 58% of the £111bn global digital ad market – it comes as no surprise that they were a major factor in merger and acquisition growth in the past decade. Of the Faangs, Netflix is the only one to have achieved its growth organically. The other four have completed more than 400 M&A deals since 2007.

Barclays believes that M&A activity will continue at a similar pace in tech and marketing services, with "private equity firms maintaining their interests in the sector" and restructuring at ad groups like WPP "presenting a range of opportunities".

The report contains good news for clients concerned about opaque practices in digital advertising. Charging for marketing services is becoming more transparent as traditional retainer-based client-agency relationships are being replaced by more project-based activity, leading to more straightforward pricing models.

Barclays earmarked out-of-home advertising as an area of great potential, citing 2018 as the year in which digital OOH overtook traditional OOH for the first time – digital panels now reach 65% of the UK audience. The bank envisages expanding availability of digital signage and the growth in volume and sophistication of smart data enabling brands to deploy "highly personalised targeting tied to specific locations".

The advance of tech such as artificial intelligence, augmented reality and machine learning could transform advertising, the report says, but cautions that commercial applications are in their infancy and "more work has to be done to bring them into the mainstream".

The report was written by Barclays Corporate Banking's Sean Duffy, head of technology, media and telecoms, and Lorraine Ruckstuhl, industry director of technology, media and telecoms.

Duffy said: "Fintech grabs all the headlines, which is understandable given the number of fintechs doing amazing things and delivering spectacular results. It feels like adtech is slightly pushed to the sidelines, which is a mistake as it is transforming advertising and can be another real growth engine for the UK economy.

"Adtech is already helping UK businesses compete on the global stage and will continue to allow brands to market themselves more effectively as further technology advances are harnessed."

Contributors to the report included James Booth, chief executive of Scoota, Justin Cochrane, chief executive of Clear Channel UK, Rob Pierre, chief executive of Jellyfish, Ben Rudman, chief operating officer of Be Heard and Luke Smith, chief executive of Croud.

Source: Ben Bold, Campaign