The Verified Marketplace will be a joint sales platform for a single type of video ad inventory, called "outstream" video.
Outstream video ads are standalone ad units and are often placed within a news article between paragraphs of editorial text.
"The reason we are focusing on outstream is we believe it’s the fastest growing video format," Hamish Nicklin, the chief revenue officer of Guardian News & Media, said.
Outstream was worth £363m or 52% of the online video ad market in the first half of 2017 in the UK, according to the IAB, which said video revenue overtook that of banner ads this year.
Advertisers will be able to buy outstream video ads on The Verified Marketplace through News UK’s UnrulyX platform.
The publishers say The Verified Marketplace will be a "trusted, brand-safe" platform for "high quality" video advertising that can reach a potential audience of 39.4 million unique users across four national titles: News UK’s The Sun and The Times, The Guardian and the Daily Telegraph.
Independent measurement companies Integral Ad Science and MOAT will verify the video ads on The Verified Marketplace.
The fact that News UK, Guardian News & Media and Telegraph Media Group have managed to strike an agreement is a small but important breakthrough.
The newspaper groups spent more than a year on a failed effort to pool their ad sales in an initiative that was called Project Juno and renamed Rio and then Arena.
They hoped that joint ad sales could stem declining revenues and make it easier for agencies and advertisers to buy from news brands, but the project collapsed as first DMGT and then Trinity Mirror and TMG pulled out.
Those involved in The Verified Marketplace believe it is evidence that something has been salvaged from Project Arena and offers hope of further collaboration.
"We’re excited about it because it shows the market that we’ve listened," Nicklin said, acknowledging that agencies want the buying process to be easier.
"We think it’s a good product, we know outstream video is growing, we know brand safety continues to be an issue, and it shows news brands can collaborate. It bodes well for the future."
Nicklin played down suggestions that consumers might skip or pass over outstream ads. "The fact that outstream is the fastest-growing video channel tells me advertisers certainly believe in it," he said.
Jason Trout, Unruly’s managing director of EMEA, said The Verified Marketplace "will deliver high-quality ad formats and peace of mind to advertisers at a time when trust in the programmatic eco-system has never been lower, solving the problem of scarcity of genuinely premium, verifiably brand safe and viewable video environments".
Robin O’Neill, manager director of digital trading at Group M, welcomed The Verified Marketplace, saying it meets two important criteria as it offers "professionally produced, high quality content for our clients’ advertising campaigns" and " the ability to independently verify delivery and performance of campaigns".
Nicklin said the news publishers had learnt from the Juno-Rio-Arena talks on collaboration that it was easier to take small "chunks", rather than "chewing the animal in one go".
He added: "We know collaboration is what the market wants."